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Chapter 16

Additional Library Utilities

WHAT’S IN THIS CHAPTER

How you can use std::function for function pointers

How to work with compile time rational numbers

How to work with time

How to generate random numbers

What tuples are and how to use them

C++11 adds a lot more functionality to the C++ standard library not described in previous chapters. These additional library features are combined and explained in this chapter because they don’t fit anywhere else.

STD::FUNCTION

std::function, defined in the header file, can be used to create a type that can point to a function, a function object, or a lambda expression; basically anything that is callable. It can be used as a function pointer or as a parameter for a function to implement callbacks. The template parameters for the std::function template look a bit different than most template parameters. Its syntax is as follows:

std::function

R is the return value type of the function and ArgTypes is a comma-separated list of argument types for the function.

The following example demonstrates how to use std::function to implement a function pointer. It creates a function pointer f1 to point to the function func(). Once f1 is defined, you can call func() by using the name func or f1:

void func(int num, const string& str)

{

cout << "func(" << num << ", " << str << ")" << endl;

}

int main()

{

function f1 = func;

f1(1, "test");

return 0;

}

Code snippet from Function\function.cpp

Of course, in the preceding example it is possible to use the C++11 auto keyword, which removes the need for you to specify the exact type of f1. The following works exactly the same and is much shorter:

auto f1 = func;

Code snippet from Function\function.cpp

Since std::function types behave as function pointers, they can be passed to STL algorithms as shown in the following example using the count_if() algorithm. STL algorithms are discussed in Chapter 13.

bool isEven(int num)

{

return num % 2 == 0;

}

int main()

{

vector vec;

for (int i = 0; i < 10; ++i)

vec.push_back(i);

auto f2 = isEven; // f2 will be of type function

int cnt = count_if(vec.cbegin(), vec.cend(), f2);

cout << cnt << " even numbers" << endl;

return 0;

}

Code snippet from Function\function_count_if.cpp

After the preceding examples, you might think that std::function is not really useful; but, where std::function really shines, is accepting a function pointer as argument to your own function. The following example defines a function called process(), which accepts a reference to a vector and a std::function. The process() function will iterate over all the elements in the given vector and will call the given function f for each element. You can think of the parameter f as a callback.

The print() function prints a given element to the console. The main() function first creates a vector of integers and populates it. It then calls the process() function with a function pointer to print(). The result will be that each element in the vector is printed.

The last part of the main() function demonstrates that you can also pass a lambda expression for the std::function parameter of the process() function, and that’s the power of std::function. You cannot get this same functionality by using a function pointer typedef.

void process(const vector& vec, function f)

{

for (auto& i : vec)

f(i);

}

void print(int num)

{

cout << num << " ";

}

int main()

{

vector vec;

for (int i = 0; i < 10; ++i)

vec.push_back(i);

process(vec, print);

cout << endl;

int sum = 0;

process(vec, [&sum](int num){sum += num;});

cout << "sum = " << sum << endl;

return 0;

}

Code snippet from Function\function_callback.cpp

The output of this example is as follows:

0 1 2 3 4 5 6 7 8 9

sum = 45

RATIOS

The C++11 Ratio library allows you to exactly represent any finite rational number that you can use at compile time. Everything is defined