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.
DeclarationFollowing 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.
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 18Member 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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