Discovering Modern C++. An Intensive Course for Scientists, Engineers, and Programmers(2016)

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Even if you are sure that a property obviously holds for your implementation, write an assertion. Sometimes the system does not behave precisely as we assumed, or the compiler might be buggy (extremely rare but possible), or we did something slightly different from what we intended originally. No matter how much we reason and how carefully we implement, sooner or later one assertion may be raised. In the case that there are so many properties that the actual functionality gets cluttered by the tests, one can outsource the tests into another function.

Responsible programmers implement large sets of tests. Nonetheless, this is no guarantee that the program works under all circumstances. An application can run for years like a charm and one day it crashes. In this situation, we can run the application in debug mode with all the assertions enabled, and in most cases they will be a great help to find the reason for the crash. However, this requires that the crashing situation is reproducible and that the program in slower debug mode reaches the critical section in reasonable time.

1.6.2 Exceptions

In the preceding section, we looked at how assertions help us to detect programming errors. However, there are many critical situations that we cannot prevent even with the smartest programming, like files that we need to read but which are deleted. Or our program needs more memory than is available on the actual machine. Other problems are preventable in theory but the practical effort is disproportionally high, e.g., to check whether a matrix is regular is feasible but might be as much or more work than the actual task. In such cases, it is usually more efficient to try to accomplish the task and check for Exceptions along the way.

1.6.2.1 Motivation

Before illustrating the old-style error handling, we introduce our anti-hero Herbert⁸ who is an ingenious mathematician and considers programming a necessary evil for demonstrating how magnificently his algorithms work. He learned to program like a real man and is immune to the newfangled nonsense of modern programming.

8. To all readers named Herbert: Please accept our honest apology for having picked your name.

His favorite approach to deal with computational problems is to return an error code (like the main function does). Say we want to read a matrix from a file and check whether the file is really there. If not, we return an error code of 1:

int read_matrix_file(const char* fname, ...)
{
fstream f(fname);
if (!f.is_open())
return 1;
...
return 0;
}

So, we checked for everything that can go wrong and informed the caller with the appropriate error code. This is fine when the caller evaluated the error and reacted appropriately. But what happens when the caller simply ignores our return code? Nothing! The program keeps going and might crash later on absurd data or even worse produce nonsensical results that careless people might use to build cars or planes. Of course, car and plane builders are not that careless, but in more realistic software even careful people cannot have an eye on each tiny detail.

Nonetheless, bringing this point across to programming dinosaurs like Herbert might not convince them: “Not only are you dumb enough to pass in a non-existing file to my perfectly implemented function, then you do not even check the return code. You do everything wrong, not me.”

Another disadvantage of the error codes is that we cannot return our computational results and have to pass them as reference arguments. This prevents us from building expressions with the result. The other way around is to return the result and pass the error code as a (referred) function argument which is not much less cumbersome.

1.6.2.2 Throwing

The better approach is to throw an exception:

matrix read_matrix_file(const char* fname, ...)
{
fstream f(fname);
if (!f.is_open())
throw "Cannot open file.";
...
}

In this version, we throw an exception. The calling application is now obliged to react on it—otherwise the program crashes.

The advantage of exception handling over error codes is that we only need to bother with a problem where we can handle it. For instance, in the function that called read_matrix_file it might not be possible to deal with a non-existing file. In this case, the code is implemented as there is no exception thrown. So, we do not need to obfuscate our program with returning error codes. In the case of an exception, it is passed up to the appropriate exception handling. In our scenario, this handling might be contained in the GUI where a new file is requested from the user. Thus, exceptions lead at the same time to more readable sources and more reliable error handling.

C++ allows us to throw everything as an exception: strings, numbers, user types, et cetera. However, to deal with the exceptions properly it is better to define exception types or to use those from the standard library:

struct cannot_open_file {};

void read_matrix_file(const char* fname, ...)
{
fstream f(fname);
if(!f.is_open())
throw cannot_open_file{};
...
}

Here, we introduced our own exception type. In Chapter 2, we will explain in detail how classes can be defined. In the example above, we defined an empty class that only requires opening and closing brackets followed by a semicolon. Larger projects usually establish an entire hierarchy of exception types that are often derived (Chapter 6) from std::exception.

1.6.2.3 Catching

To react to an exception, we have to catch it. This is done in a try-catch-block:

try {
...
} catch (e1_type& e1)
{ ...
} catch (e2_type& e2) { ... }

Wherever we expect a problem that we can solve (or at least do something about), we open a try-block. After the closing braces, we can catch exceptions and start a rescue depending on the type of the exception and possibly on its value. It is recommended to catch exceptions by reference [45, Topic 73], especially when polymorphic types (Definition 6–1 in §6.1.3) are involved. When an exception is thrown, the first catch-block with a matching type is executed. Further catch-blocks of the same type (or sub-types; §6.1.1) are ignored. A catch-block with an ellipsis, i.e., three dots literally, catches all exceptions:

try { ...
} catch (e1_type& e1) { ... }
catch (e2_type& e2) { ... }
catch (...) { // deal with all other exceptions
}

Obviously, the catch-all handler should be the last one.

If nothing else, we can catch the exception to provide an informative error message before terminating the program:

try {
A= read_matrix_file("does_not_exist.dat");
} catch (cannot_open_file& e) {
cerr "Hey guys, your file does not exist! I'm out.\n";
exit(EXIT_FAILURE);
}

Once the exception is caught, the problem is considered to be solved and the execution continues after the catch-block(s). To terminate the execution, we used exit from the header <cstdlib>. The function exit ends the execution even when we are not in the main function. It should only be used when further execution is too dangerous and there is no hope that the calling functions have any cure for the exception either.

Alternatively we can continue after the complaint or a partial rescue action by rethrowing the exception which might be dealt with later:

try {
A= read_matrix_file("does_not_exist.dat");
} catch (cannot_open_file& e) {
cerr "O my gosh, the file is not there! Please caller help me.\n";
throw e;
}

In our case, we are already in the main function and there is nothing else on the call stack to catch our exception. For rethrowing the current one, there exists a shorter notation:

} catch (cannot_open_file&) {
...
throw;
}

This shortcut is preferred since it is less error-prone and shows more clearly that we rethrow the original exception. Ignoring an exception is easily implemented by an empty block:

} catch (cannot_open_file&) {} // File is rubbish, keep going

So far, our exception handling did not really solve our problem of missing a file. If the file name is provided by a user, we can pester him/her until we get one that makes us happy:

bool keep_trying= true;
do {
char fname[80]; // std::string is better
cout "Please enter the file name: ";
cin fname;
try {
A= read_matrix_file(fname);
...
keep_trying= false;
} catch (cannot_open_file& e) {
cout "Could not open the file. Try another one!\n";
} catch (...)
cout "Something is fishy here. Try another file!\n";
}
} while (keep_trying);

When we reach the end of the try-block, we know that no exception was thrown and we can call it a day. Otherwise, we land in one of the catch-blocks and keep_trying remains true.

A great advantage of exceptions is that issues that cannot be handled in the context where they are detected can be postponed for later. An example from the author’s practice concerned an LU factorization. It cannot be computed for a singular matrix. There is nothing we can do about it. However, in the case that the factorization was part of an iterative computation, we were able to continue the iteration somehow without that factorization. Although this would be possible with traditional error handling as well, exceptions allow us to implement it much more readably and elegantly. We can program the factorization for the regular case and when we detect the singularity, we throw an exception. Then it is up to the caller how to deal with the singularity in the respective context—if possible.

1.6.2.4 Who Throws?

Already C++03 allowed specifying which types of exceptions can be thrown from a function. Without going into details, these specifications turned out to be not very useful and are deprecated now.

C++11 added a new qualification for specifying that no exceptions must be thrown out of the function, e.g.:

double square_root(double x) noexcept { ... }

The benefit of this qualification is that the calling code never needs to check for thrown exceptions after square_root. If an exception is thrown despite the qualification, the program is terminated.

In templated functions, it can depend on the argument type(s) whether an exception is thrown. To handle this properly, noexcept can depend on a compile-time condition; see Section 5.2.2.

Whether an assertion or an exception is preferable is not an easy question and we have no short answer to it. The question will probably not bother you now. We therefore postpone the discussion to Section A.2.6 and leave it to you when you read it.

1.6.3 Static Assertions

Program errors that can already be detected during compilation can raise a static_assert. In this case, an error message is emitted and the compilation stopped. An example would not make sense at this point and we postpone it till Section 5.2.5.

1.7 I/O

C++ uses a convenient abstraction called streams to perform I/O operations in sequential media such as screens or keyboards. A stream is an object where a program can either insert characters or extract them. The standard C++ library contains the header <iostream> where the standard input and output stream objects are declared.

1.7.1 Standard Output

By default, the standard output of a program is written to the screen, and we can access it with the C++ stream named cout. It is used with the insertion operator which is denoted by (like left shift). We have already seen that it may be used more than once within a single statement. This is especially useful when we want to print a combination of text, variables, and constants, e.g.:

cout "The square root of " x " is " sqrt(x) endl;

with an output like

The square root of 5 is 2.23607

endl produces a newline character. An alternative representation of endl is the character \n. For the sake of efficiency, the output may be buffered. In this regard, endl and \n differ: the former flushes the buffer while the latter does not. Flushing can help us when we are debugging (without a debugger) to find out between which outputs the program crashes. In contrast, when a large amount of text is written to files, flushing after every line slows down I/O considerably.

Fortunately, the insertion operator has a relatively low priority so that arithmetic operations can be written directly:

std::cout "11 * 19 = " 11 * 19 std::endl;

All comparisons and logical and bitwise operations must be grouped by surrounding parentheses. Likewise the conditional operator:

std::cout (age > 65 ? "I'm a wise guy\n" : "I am still half-baked.\n");

When we forget the parentheses, the compiler will remind us (offering us an enigmatic message to decipher).

1.7.2 Standard Input

The standard input device is usually the keyboard. Handling the standard input in C++ is done by applying the overloaded operator of extraction on the cin stream:

int age;
std::cin age;

std::cin reads characters from the input device and interprets them as a value of the variable type (here int) it is stored to (here age). The input from the keyboard is processed once the RETURN key has been pressed.

We can also use cin to request more than one data input from the user:

std::cin width length;

which is equivalent to

std::cin width;
std::cin length;

In both cases the user must provide two values: one for width and another for length. They can be separated by any valid blank separator: a space, a tab character, or a newline.

1.7.3 Input/Output with Files

C++ provides the following classes to perform input and output of characters from/to files:

We can use file streams in the same fashion as cin and cout, with the only difference that we have to associate these streams with physical files. Here is an example:

#include <fstream>

int main ()
{
std::ofstream myfile;
square_file.open("squares.txt");
for (int i= 0; i < 10; ++i)
square_file i "^2 = " i*i std::endl;
square_file.close();
}

This code creates a file named squares.txt (or overwrites it if it already exists) and writes a sentence to it—like we write to cout. C++ establishes a general stream concept that is satisfied by an output file and by std::cout. This means we can write everything to a file that we can write to std::cout and vice versa. When we define operator for a new type, we do this once for ostream (Section 2.7.3) and it will work with the console, with files, and with any other output stream.

Alternatively, we can pass the file name as an argument to the constructor of the stream to open the file implicitly. The file is also implicitly closed when square_file goes out of scope,⁹ in this case at the end of the main function. The short version of the previous program is

9. Thanks to the powerful technique named RAII, which we will discuss in Section 2.4.2.1.

#include <fstream>

int main ()
{
std::ofstream square_file("squares.txt");
for (int i= 0; i < 10; ++i)
square_file i "^2 = " i*i std::endl;
}

We prefer the short form (as usual). The explicit form is only necessary when the file is first declared and opened later for some reason. Likewise, the explicit close is only needed when the file should be closed before it goes out of scope.

1.7.4 Generic Stream Concept

Streams are not limited to screens, keyboards, and files; every class can be used as a stream when it is derived¹⁰ from istream, ostream, or iostream and provides implementations for the functions of those classes. For instance, Boost.Asio offers streams for TCP/IP and Boost.IOStream as alternatives to the I/O above. The standard library contains a stringstream that can be used to create a string from any kind of printable type. stringstream’s method str() returns the stream’s internal string.

10. How classes are derived is shown in Chapter 6. Let us here just take notice that being an output stream is technically realized by deriving it from std::ostream.

We can write output functions that accept every kind of output stream by using a mutable reference to ostream as an argument:

#include <iostream>
#include <fstream>
#include <sstream>

void write_something (std::ostream& os)
{
os "Hi stream, did you know that 3 * 3 = " 3 * 3 std::endl;
}

int main (int argc, char* argv[])
{
std::ofstream myfile("example.txt");
std::stringstream mysstream;

write_something(std::cout);
write_something(myfile);
write_something(mysstream);

std::cout "mysstream is: " mysstream.str(); // newline contained
}

Likewise, generic input can be implemented with istream and read/write I/O with iostream.

1.7.5 Formatting

c++03/formatting.cpp

I/O streams are formatted by so-called I/O manipulators which are found in the header file <iomanip>. By default, C++ only prints a few digits of floating-point numbers. Thus, we increase the precision:

double pi= M_PI;
cout "pi is " pi '\n';
cout "pi is " setprecision(16) pi '\n';

and yield a more accurate number:

pi is 3.14159
pi is 3.141592653589793

In Section 4.3.1, we will show how the precision can be adjusted to the type’s representable number of digits.

When we write a table, vector, or matrix, we need to align values for readability. Therefore, we next set the width of the output:

cout "pi is " setw(30) pi '\n';

This results in

pi is 3.141592653589793

setw changes only the next output while setprecision affects all following (numerical) outputs, like the other manipulators. The provided width is understood as a minimum, and if the printed value needs more space, our tables will get ugly.

We can further request that the values be left aligned, and the empty space be filled with a character of our choice, say, -:

cout "pi is " setfill('-') left
setw(30) pi '\n';

yielding

pi is 3.141592653589793 - - - - - - - - - - - - -

Another way of formatting is setting the flags directly. Some less frequently used format options can only be set this way, e.g., whether the sign is shown for positive values as well. Furthermore, we force the “scientific” notation in the normalized exponential representation:

cout.setf(ios_base::showpos);
cout "pi is " scientific pi '\n';

resulting in

pi is +3.1415926535897931e+00

Integer numbers can be represented in octal and hexadecimal base by

cout "63 octal is " oct 63 ".\n";
cout "63 hexadecimal is " hex 63 ".\n";
cout "63 decimal is " dec 63 ".\n";

with the expected output:

63 octal is 77.
63 hexadecimal is 3f.
63 decimal is 63.

Boolean values are by default printed as integers 0 and 1. On demand, we can present them as true and false:

cout "pi < 3 is " (pi < 3) '\n';
cout "pi < 3 is " boolalpha (pi < 3) '\n';

Finally, we can reset all the format options that we changed:

int old_precision= cout.precision ();
cout setprecision(16)
...
cout.unsetf(ios_base::adjustfield | ios_base::basefield
| ios_base::floatfield | ios_base::showpos | ios_base::boolalpha);
cout.precision(old_precision);

Each option is represented by a bit in a status variable. To enable multiple options, we can combine their bit patterns with a binary OR.

1.7.6 Dealing with I/O Errors

To make one thing clear from the beginning: I/O in C++ is not fail-safe (let alone idiot-proof). Errors can be reported in different ways and our error handling must comply to them. Let us try the following example program:

int main ()
{
std::ifstream infile("some_missing_file.xyz");

int i;
double d;
infile i d;

std::cout "i is " i ", d is " d '\n';
infile.close();
}

Although the file does not exist, the opening operation does not fail. We can even read from the non-existing file and the program goes on. Needless to say that the values in i and d are nonsense:

i is 1, d is 2.3452e-310

By default, the streams do not throw exceptions. The reason is historical: they are older than the exceptions and later the behavior was kept to not break software written in the meantime.

To be sure that everything went well, we have to check error flags, in principle, after each I/O operation. The following program asks the user for new file names until a file can be opened. After reading its content, we check again for success:

int main ()
{
std::ifstream infile;
std::string filename{"some_missing_file.xyz"};
bool opened= false;
while (!opened) {
infile.open(filename);
if (infile.good()) {
opened= true;
} else {
std::cout "The file '" filename
"' doesn't exist, give a new file name: ";
std::cin filename;
}
}
int i;
double d;
infile i d;

if (infile.good())
std::cout "i is " i ", d is " d '\n';
else
std::cout "Could not correctly read the content.\n";
infile.close();
}

You can see from this simple example that writing robust applications with file I/O can create some work.

If we want to use exceptions, we have to enable them during run time for each stream:

cin.exceptions(ios_base::badbit | ios_base::failbit);
cout.exceptions(ios_base::badbit | ios_base::failbit);

std::ifstream infile("f.txt");
infile.exceptions(ios_base::badbit | ios_base::failbit);

The streams throw an exception every time an operation fails or when they are in a “bad” state. Exceptions could be thrown at (unexpected) file end as well. However, the end of file is more conveniently handled by testing (e.g., while (!f.eof())).

In the example above, the exceptions for infile are only enabled after opening the file (or the attempt thereof). For checking the opening operation, we have to create the stream first, then turn on the exceptions and finally open the file explicitly. Enabling the exceptions gives us at least the guarantee that all I/O operations went well when the program terminates properly. We can make our program more robust by catching exceptions that might be thrown.

The exceptions in file I/O only protect us partially from making errors. For instance, the following small program is obviously wrong (types don’t match and numbers aren’t separated):

void with_io_exceptions(ios& io)
{ io.exceptions(ios_base::badbit | ios_base::failbit); }

int main ()
{
std::ofstream outfile;
with_io_exceptions(outfile);
outfile.open("f.txt");

double o1= 5.2, o2= 6.2;
outfile o1 o2 std::endl; // no separation
outfile.close();

std::ifstream infile;
with_io_exceptions(infile);
infile.open("f.txt");

int i1, i2;
char c;
infile i1 c i2; // mismatching types
std::cout "i1 = " i1 ", i2 = " i2 "\n";
}

Nonetheless, it does not throw exceptions and fabricates the following output:

i1 = 5, i2 = 26

As we all know, testing does not prove the correctness of a program. This is even more obvious when I/O is involved. Stream input reads the incoming characters and passes them as values of the appropriate variable type, e.g., int when setting i1. It stops at the first character that cannot be part of the value, first at the . for the int value i1. If we read another int afterward, it would fail because an empty string cannot be interpreted as an int value. But we do not; instead we read a char next to which the dot is assigned. When parsing the input for i2 we find first the fractional part from o1 and then the integer part from o1 before we get a character that cannot belong to an int value.

Unfortunately, not every violation of the grammatical rules causes an exception in practice: .3 parsed as an int yields zero (while the next input probably fails); -5 parsed as an unsigned results in 4294967291 (when unsigned is 32 bits long). The narrowing principle apparently has not found its way into I/O streams yet (if it ever will for backward compatibility’s sake).

At any rate, the I/O part of an application needs utter attention. Numbers must be separated properly (e.g., by spaces) and read with the same type as they were written. When the output contains branches such that the file format can vary, the input code is considerably more complicated and might even be ambiguous.

There are two other forms of I/O we want to mention: binary and C-style I/O. The interested reader will find them in Sections A.2.7 and A.2.8, respectively. You can also read this later when you need it.

1.8 Arrays, Pointers, and References

1.8.1 Arrays

The intrinsic array support of C++ has certain limitations and some strange behaviors. Nonetheless, we feel that every C++ programmer should know it and be aware of its problems.

An array is declared as follows:

int x[10];

The variable x is an array with 10 int entries. In standard C++, the size of the array must be constant and known at compile time. Some compilers (e.g., gcc) support run-time sizes.

Arrays are accessed by square brackets: x[i] is a reference to the i-th element of x. The first element is x[0]; the last one is x[9]. Arrays can be initialized at the definition:

float v[]= {1.0, 2.0, 3.0}, w[]= {7.0, 8.0, 9.0};

In this case, the array size is deduced.

The list initialization in C++11 cannot be narrowed any further. This will rarely make a difference in practice. For instance, the following:

int v[]= {1.0, 2.0, 3.0}; // Error in C++11: narrowing

was legal in C++03 but not in C++11 since the conversion from a floating-point literal to int potentially loses precision. However, we would not write such ugly code anyway.

Operations on arrays are typically performed in loops; e.g., to compute x = v – 3w as a vector operation is realized by

float x[3];
for (int i= 0; i < 3; ++i)
x[i]= v[i] - 3.0 * w[i];

We can also define arrays of higher dimensions:

float A[7][9]; // a 7 by 9 matrix
int q[3][2][3]; // a 3 by 2 by 3 array

The language does not provide linear algebra operations upon the arrays. Implementations based on arrays are inelegant and error-prone. For instance, a function for a vector addition would look like this:

void vector_add(unsigned size, const double v1[], const double v2[],
double s[])
{
for (unsigned i= 0; i < size; ++i)
s[i]= v1[i] + v2[i];
}

Note that we passed the size of the arrays as first function parameter whereas array parameters don’t contain size information.¹¹ In this case, the function’s caller is responsible for passing the correct size of the arrays:

11. When passing arrays of higher dimensions, only the first dimension can be open while the others must be known during compilation. However, such programs get easily nasty and we have better techniques for it in C++.

int main ()
{
double x[]= {2, 3, 4}, y[]= {4, 2, 0}, sum[3];
vector_add(3, x, y, sum);
...
}

Since the array size is known during compilation, we can compute it by dividing the byte size of the array by that of a single entry:

vector_add(sizeof x / sizeof x[0], x, y, sum);

With this old-fashioned interface, we are also unable to test whether our arrays match in size. Sadly enough, C and Fortran libraries with such interfaces where size information is passed as function arguments are still realized today. They crash at the slightest user mistake, and it can take enormous efforts to trace back the reasons for crashing. For that reason, we will show in this book how we can realize our own math software that is easier to use and less prone to errors. Hopefully, future C++ standards will come with more higher mathematics, especially a linear-algebra library.

Arrays have the following two disadvantages:

• Indices are not checked before accessing an array, and we can find ourselves outside the array and the program crashes with segmentation fault/violation. This is not even the worst case; at least we see that something goes wrong. The false access can also mess up our data; the program keeps running and produces entirely wrong results with whatever consequence you can imagine. We could even overwrite the program code. Then our data is interpreted as machine operations leading to any possible nonsense.

• The size of the array must be known at compile time.¹² For instance, we have an array stored to a file and need to read it back into memory:

12. Some compilers support run-time values as array sizes. Since this is not guaranteed with other compilers one should avoid this in portable software. This feature was considered for C++14 but its inclusion postponed as not all subtleties were entirely clarified.

ifstream ifs("some_array.dat");
ifs size;
float v[size]; // Error: size not known at compile time

This does not work because the size needs to be known during compilation.

The first problem can only be solved with new array types and the second one with dynamic allocation. This leads us to pointers.

1.8.2 Pointers

A pointer is a variable that contains a memory address. This address can be that of another variable that we can get with the address operator (e.g., &x) or dynamically allocated memory. Let’s start with the latter as we were looking for arrays of dynamic size.

int* y= new int[10];

This allocates an array of 10 int. The size can now be chosen at run time. We can also implement the vector reading example from the previous section:

ifstream ifs("some_array.dat");
int size;
ifs size;
float* v= new float[size];
for (int i= 0; i < size; ++i)
ifs v[i];

Pointers bear the same danger as arrays: accessing data out of range which can cause program crashes or silent data invalidation. When dealing with dynamically allocated arrays, it is the programmer’s responsibility to store the array size.

Furthermore, the programmer is responsible for releasing the memory when not needed anymore. This is done by

delete[] v;

Since arrays as function parameters are treated internally as pointers, the vector_add function from page 47 works with pointers as well:

int main (int argc, char* argv[])
{
double *x= new double[3], *y= new double[3], *sum= new double[3];
for (unsigned i= 0; i < 3; ++i)
x[i]= i+2, y[i]= 4-2*i;
vector_add(3, x, y, sum);
...
}

With pointers, we cannot use the sizeof trick; it would only give us the byte size of the pointer itself which is of course independent of the number of entries. Other than that, pointers and arrays are interchangeable in most situations: a pointer can be passed as an array argument (as in the previous listing) and an array as a pointer argument. The only place where they are really different is the definition: whereas defining an array of size n reserves space for n entries, defining a pointer only reserves the space to hold an address.

Since we started with arrays, we took the second step before the first one regarding pointer usage. The simple use of pointers is allocating one single data item:

int* ip= new int;

Releasing this memory is performed by

delete ip;

Note the duality of allocation and release: the single-object allocation requires a single-object release and the array allocation demands an array release. Otherwise the run-time system will handle the deallocation incorrectly and most likely crash at this point. Pointers can also refer to other variables:

int i= 3;
int* ip2= &i;

The operator & takes an object and returns its address. The opposite operator is * which takes an address and returns an object:

int j= *ip2;

This is called Dereferencing. Given the operator priorities and the grammar rules, the meaning of the symbol * as dereference or multiplication cannot be confused—at least not by the compiler.

Pointers that are not initialized contain a random value (whatever bits are set in the corresponding memory). Using uninitialized pointers can cause any kind of error. To say explicitly that a pointer is not pointing to something, we should set it to

int* ip3= nullptr; // >= C++11
int* ip4{}; // ditto

or in old compilers:

int* ip3= 0; // better not in C++11 and later
int* ip4= NULL; // ditto

The address 0 is guaranteed never to be used for applications, so it is safe to indicate this way that the pointer is empty (not referring to something). Nonetheless the literal 0 does not clearly convey its intention and can cause ambiguities in function overloading. The macro NULL is not better: it just evaluates to 0. C++11 introduces nullptr as a keyword for a pointer literal. It can be assigned to or compared with all pointer types. As it cannot be confused with other types and is self-explanatory, it is preferred over the other notations. The initialization with an empty braced list also sets a nullptr.

The biggest danger of pointers is Memory Leaks. For instance, our array y became too small and we want to assign a new array:

int* y= new int[15];

We can now use more space in y. Nice. But what happened to the memory that we allocated before? It is still there but we have no access to it anymore. We cannot even release it because this requires the address too. This memory is lost for the rest of our program execution. Only when the program is finished will the operating system be able to free it. In our example, we only lost 40 bytes out of several gigabytes that we might have. But if this happens in an iterative process, the unused memory grows continuously until at some point the whole (virtual) memory is exhausted.

Even if the wasted memory is not critical for the application at hand, when we write high-quality scientific software, memory leaks are unacceptable. When many people are using our software, sooner or later somebody will criticize us for it and eventually discourage other people from using our software. Fortunately, there are tools to help you to find memory leaks, as demonstrated in Section B.3.

The demonstrated issues with pointers are not intended as fun killers. And we do not discourage the use of pointers. Many things can only be achieved with pointers: lists, queues, trees, graphs, et cetera. But pointers must be used with utter care to avoid all the really severe problems mentioned above.

There are three strategies to minimize pointer-related errors:

Use standard containers: from the standard library or other validated libraries. std::vector from the standard library provides us all the functionality of dynamic arrays, including resizing and range check, and the memory is released automatically.

Encapsulate: dynamic memory management in classes. Then we have to deal with it only once per class.¹³ When all memory allocated by an object is released when the object is destroyed, then it does not matter how often we allocate memory. If we have 738 objects with dynamic memory, then it will be released 738 times. The memory should be allocated in the object construction and deallocated in its destruction. This principle is called Resource Allocation Is Initialization (RAII). In contrast, if we called new 738 times, partly in loops and branches, can we be sure that we have called delete exactly 738 times? We know that there are tools for this but these are errors that are better to prevent than to fix.¹⁴ Of course, the encapsulation idea is not idiot-proof but it is much less work to get it right than sprinkling (raw) pointers all over our program. We will discuss RAII in more detail in Section 2.4.2.1.

13. It is safe to assume that there are many more objects than classes; otherwise there is something wrong with the entire program design.

14. In addition, the tool only shows that the current run had no errors but this might be different with other input.

Use smart pointers: which we will introduce in the next section (§1.8.3).

Pointers serve two purposes:

• Referring to objects; and

• Managing dynamic memory.

The problem with so-called Raw Pointers is that we have no notion whether a pointer is only referring to data or also in charge of releasing the memory when it is not needed any longer. To make this distinction explicit at the type level, we can use Smart Pointers.

1.8.3 Smart Pointers

Three new smart-pointer types are introduced in C++11: unique_ptr, shared_ptr, and weak_ptr. The already existing smart pointer from C++03 named auto_ptr is generally considered as a failed attempt on the way to unique_ptr since the language was not ready at the time. It should not be used anymore. All smart pointers are defined in the header <memory>. If you cannot use C++11 features on your platform (e.g., in embedded programming), the smart pointers in Boost are a decent replacement.

1.8.3.1 Unique Pointer

This pointer’s name indicates Unique Ownership of the referred data. It can be used essentially like an ordinary pointer:

#include <memory>

int main ()
{
unique_ptr<double> dp{new double};
*dp= 7;
...
}

The main difference from a raw pointer is that the memory is automatically released when the pointer expires. Therefore, it is a bug to assign addresses that are not allocated dynamically:

double d;
unique_ptr<double> dd{&d}; // Error: causes illegal deletion

The destructor of pointer dd will try to delete d.

Unique pointers cannot be assigned to other pointer types or implicitly converted. For referring to the pointer’s data in a raw pointer, we can use the member function get:

double* raw_dp= dp.get();

It cannot even be assigned to another unique pointer:

unique_ptr<double> dp2{dp}; // Error: no copy allowed
dp2= dp; // ditto

It can only be moved:

unique_ptr<double> dp2{move(dp)}, dp3;
dp3= move(dp2);

We will discuss move semantics in Section 2.3.5. Right now let us just say this much: whereas a copy duplicates the data, a Move transfers the data from the source to the target. In our example, the ownership of the referred memory is first passed from dp to dp2 and then to dp3. dp anddp2 are nullptr afterward, and the destructor of dp3 will release the memory. In the same manner, the memory’s ownership is passed when a unique_ptr is returned from a function. In the following example, dp3 takes over the memory allocated in f():

std::unique_ptr<double> f()
{ return std::unique_ptr<double>{new double}; }

int main ()
{
unique_ptr<double> dp3;
dp3= f();
}

In this case, move() is not needed since the function result is a temporary that will be moved (again, details in §2.3.5).

Unique pointer has a special implementation¹⁵ for arrays. This is necessary for properly releasing the memory (with delete[]). In addition, the specialization provides array-like access to the elements:

15. Specialization will be discussed in §3.6.1 and §3.6.3.

unique_ptr<double[]> da{new double[3]};
for (unsigned i= 0; i < 3; ++i)
da[i]= i+2;

In return, the operator* is not available for arrays.

An important benefit of unique_ptr is that it has absolutely no overhead over raw pointers: neither in time nor in memory.

Further reading: An advanced feature of unique pointers is to provide our own Deleter; for details see [26, §5.2.5f], [43, §34.3.1], or an online reference (e.g., cppreference.com).

1.8.3.2 Shared Pointer

As its name indicates, a shared_ptr manages memory that is used in common by multiple parties (each holding a pointer to it). The memory is automatically released as soon as no shared_ptr is referring the data any longer. This can simplify a program considerably, especially with complicated data structures. An extremely important application area is concurrency: the memory is automatically freed when all threads have terminated their access to it.

In contrast to a unique_ptr, a shared_ptr can be copied as often as desired, e.g.:

shared_ptr<double> f()
{
shared_ptr<double> p1{new double};
shared_ptr<double> p2{new double}, p3= p2;
cout "p3.use_count() = " p3.use_count() endl;
return p3;
}

int main ()
{
shared_ptr<double> p= f();
cout "p.use_count() = " p.use_count() endl;
}

In the example, we allocated memory for two double values: in p1 and in p2. The pointer p2 is copied into p3 so that both point to the same memory as illustrated in Figure 1–1.

Figure 1–1: Shared pointer in memory

We can see this from the output of use_count:

p3.use_count() = 2
p.use_count() = 1

When the function returns, the pointers are destroyed and the memory referred to by p1 is released (without ever being used). The second allocated memory block still exists since p from the main function is still referring to it.

If possible, a shared_ptr should be created with make_shared:

shared_ptr<double> p1= make_shared<double>();

Then the management and business data are stored together in memory—as shown in Figure 1–2—and the memory caching is more efficient. Since make_shared returns a shared pointer, we can use automatic type detection (§3.4.1) for simplicity:

auto p1= make_shared<double>();

Figure 1–2: Shared pointer in memory after make_shared

We have to admit that a shared_ptr has some overhead in memory and run time. On the other hand, the simplification of our programs thanks to shared_ptr is in most cases worth some small overhead.

Further reading: For deleters and other details of shared_ptr see the library reference [26, §5.2], [43, §34.3.2], or an online reference.

1.8.3.3 Weak Pointer

A problem that can occur with shared pointers is Cyclic References that impede the memory to be released. Such cycles can be broken by weak_ptrs. They do not claim ownership of the memory, not even a shared one. At this point, we only mention them for completeness and suggest that you read appropriate references when their need is established: [26, §5.2.2], [43, §34.3.3], or cppreference.com.

For managing memory dynamically, there is no alternative to pointers. To only refer to other objects, we can use another language feature called Reference (surprise, surprise), which we introduce in the next section.

1.8.4 References

The following code introduces a reference:

int i= 5;
int& j= i;
j= 4;
std::cout "j = " j '\n';

The variable j is referring to i. Changing j will also alter i and vice versa, as in the example. i and j will always have the same value. One can think of a reference as an alias: it introduces a new name for an existing object or sub-object. Whenever we define a reference, we must directly declare what it is referring to (other than pointers). It is not possible to refer to another variable later.

So far, that does not sound extremely useful. References are extremely useful for function arguments (§1.5), for referring to parts of other objects (e.g., the seventh entry of a vector), and for building views (e.g., §5.2.3).

As a compromise between pointers and references, the new standard offers a reference_wrapper class which behaves similarly to references but avoids some of their limitations. For instance, it can be used within containers; see §4.4.2.

1.8.5 Comparison between Pointers and References

The main advantage of pointers over references is the ability of dynamic memory management and address calculation. On the other hand, references are forced to refer to existing locations.¹⁶ Thus, they do not leave memory leaks (unless you play really evil tricks), and they have the same notation in usage as the referred object. Unfortunately, it is almost impossible to construct containers of references.

16. References can also refer to arbitrary addresses but you must work harder to achieve this. For your own safety, we will not show you how to make references behave as badly as pointers.

In short, references are not fail-safe but are much less error-prone than pointers. Pointers should be only used when dealing with dynamic memory, for instance when we create data structures like lists or trees dynamically. Even then we should do this via well-tested types or encapsulate the pointer(s) within a class whenever possible. Smart pointers take care of memory allocation and should be preferred over raw pointers, even within classes. The pointer-reference comparison is summarized in Table 1-9.

Table 1–9: Comparison between Pointers and References

1.8.6 Do Not Refer to Outdated Data!

Function-local variables are only valid within the function’s scope, for instance:

double& square_ref(double d) // DO NOT!
{
double s= d * d;
return s;
}

Here, our function result refers the local variable s which does not exist anymore. The memory where it was stored is still there and we might be lucky (mistakenly) that it is not overwritten yet. But this is nothing we can count on. Actually, such hidden errors are even worse than the obvious ones because they can ruin our program only under certain conditions and then they are very hard to find.

Such references are called Stale References. Good compilers will warn us when we are referring to a local variable. Sadly enough, we have seen such examples in web tutorials.

The same applies to pointers:

double* square_ptr(double d) // DO NOT!
{
double s= d * d;
return &s;
}

This pointer holds a local address that has gone out of scope. This is called a Dangling Pointer.

Returning references or pointers can be correct in member functions when member data is referred to; see Section 2.6.