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Introduction
Function objects are objects specifically designed to be used with a syntax similar to that of functions. Instances of std::function can store, copy, and invoke any Callable target — functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.
Declaration
Following is the declaration for std::function.
template<class > class function;
C++11
template< class R, class... Args > class function<R(Args...)>
Parameters
-
R − result_type.
-
argument_type − T if sizeof…(Args)==1 and T is the first and only type in Args.
Example
In below example for std::function.
#include <functional>
#include <iostream>
struct Foo {
Foo(int num) : num_(num) {}
void print_add(int i) const { std::cout << num_+i << ''n''; }
int num_;
};
void print_num(int i) {
std::cout << i << ''n'';
}
struct PrintNum {
void operator()(int i) const {
std::cout << i << ''n'';
}
};
int main() {
std::function<void(int)> f_display = print_num;
f_display(-9);
std::function<void()> f_display_42 = []() { print_num(42); };
f_display_42();
std::function<void()> f_display_31337 = std::bind(print_num, 31337);
f_display_31337();
std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
const Foo foo(314159);
f_add_display(foo, 1);
std::function<int(Foo const&)> f_num = &Foo::num_;
std::cout << "num_: " << f_num(foo) << ''n'';
using std::placeholders::_1;
std::function<void(int)> f_add_display2= std::bind( &Foo::print_add, foo, _1 );
f_add_display2(2);
std::function<void(int)> f_add_display3= std::bind( &Foo::print_add, &foo, _1 );
f_add_display3(3);
std::function<void(int)> f_display_obj = PrintNum();
f_display_obj(18);
}
The sample output should be like this −
-9 42 31337 314160 num_: 314159 314161 314162 18
Member functions
| Sr.No. | Member functions | Definition |
|---|---|---|
| 1 | (constructor) | It is used to construct a new std::function instance |
| 2 | (destructor) | It is used to destroy a std::function instance |
| 3 | operator= | It is used to assign a new target |
| 4 | swap | It is used to swap the contents |
| 5 | assign | It is used to assign a new target |
| 6 | operator bool | It is used to check if a valid target is contained |
| 7 | operator() | It is used to invoke the target |
Non-member functions
| Sr.No. | Non-member functions | Definition |
|---|---|---|
| 1 | std::swap | It specializes the std::swap algorithm |
| 2 | operator== operator!= | It compares an std::function with nullptr |
Operator classes
| Sr.No. | Operator classes | Definition |
|---|---|---|
| 1 | bit_and | It is a bitwise AND function object class |
| 2 | bit_or | It is a bitwise OR function object class |
| 3 | bit_xor | It is a bitwise XOR function object class |
| 3 | divides | It is a division function object class |
| 4 | equal_to | It is a function object class for equality comparison |
| 5 | greater | It is a function object class for greater-than inequality comparison |
| 6 | greater_equal | It is a function object class for greater-than-or-equal-to comparison |
| 7 | less | It is a function object class for less-than inequality comparison |
| 8 | less_equal | It is a function object class for less-than-or-equal-to comparison |
| 9 | logical_and | It is a logical AND function object class |
| 10 | logical_not | It is a logical NOT function object class |
| 11 | logical_or | It is a logical OR function object class |
| 12 | minus | It is a subtraction function object class |
| 13 | modulus | It is a modulus function object class |
| 14 | multiplies | It is a multiplication function object class |
| 15 | negate | It is a negative function object class |
| 16 | not_equal_to | It is a function object class for non-equality comparison |
| 17 | plus | It is an addition function object class |
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