std::bind

template< class F, class... Args >
/*unspecified*/ bind( F&& f, Args&&... args );

template< class R, class F, class... Args >
/*unspecified*/ bind( F&& f, Args&&... args );

The function template bind generates a forwarding call wrapper for f. Calling this wrapper is equivalent to invoking f with some of its arguments bound to args.

Parameters

Return value

A function object of unspecified type T, for which std::is_bind_expression<T>::value == true. It has the following members:

std::bind return type

Member objects

The return type of std::bind holds a member object of type std::decay<F>::type constructed from std::forward<F>(f), and one object per each of args..., of type std::decay<Arg_i>::type, similarly constructed from std::forward<Arg_i>(arg_i).

Constructors

The return type of std::bind is CopyConstructible if all of its member objects (specified above) are CopyConstructible, and is MoveConstructible otherwise. The type defines the following members:

Member type result_type

1) If F is a pointer to function or a pointer to member function, result_type is the return type of F. If F is a class type with nested typedef result_type, then result_type is F::result_type. Otherwise no result_type is defined.

2) result_type is exactly R.

Member function operator()

Given an object g obtained from an earlier call to bind, when it is invoked in a function call expression g(u1, u2, ... uM), an invocation of the stored object of type std::decay<F>::type takes place, with arguments defined as follows:

• If the argument is of type std::reference_wrapper<T> (for example, std::ref or std::cref was used in the initial call to bind), then the reference T& stored in the bound argument is passed to the invocable object.
• If std::is_bind_expression<T>::value == true (e.g., another bind subexpression was used as an argument in the initial call to bind), then that bind subexpression is invoked immediately as an lvalue object with the same cv-qualifications as that of g, and its result is passed to the invocable object. If the bind subexpression has any placeholder arguments, they are picked from u1, u2, ....
• If std::is_placeholder<T>::value != 0 (e.g., std::placeholders::_1, _2, _3, ... was used as the argument to the initial call to bind), then the argument indicated by the placeholder (u1 for _1, u2 for _2, etc) is passed to the invocable object as std::forward<Uj>(uj).
• Otherwise, the stored argument is passed to the invocable object as an lvalue with the same cv-qualification as that of g.

If some of the arguments that are supplied in the call to g() are not matched by any placeholders stored in g, the unused arguments are evaluated and discarded.

If g is volatile-qualified (i.e., its cv-qualifiers are either volatile or const volatile), the behavior is undefined.

Exceptions

Only throws if construction of std::decay<F>::type from std::forward<F>(f) throws, or any of the constructors for std::decay<Arg_i>::type from the corresponding std::forward<Arg_i>(arg_i) throws where Arg_i is the ith type and arg_i is the ith argument in Args... args.

Notes

As described in Callable, when invoking a pointer to non-static member function or pointer to non-static data member, the first argument has to be a reference or pointer (including, possibly, smart pointer such as std::shared_ptr and std::unique_ptr) to an object whose member will be accessed.

The arguments to bind are copied or moved, and are never passed by reference unless wrapped in std::ref or std::cref.

Duplicate placeholders in the same bind expression (multiple _1's for example) are allowed, but the results are only well defined if the corresponding argument (u1) is an lvalue or non-movable rvalue.

Example

#include <random>
#include <iostream>
#include <memory>
#include <functional>
 
void f(int n1, int n2, int n3, const int& n4, int n5)
{
    std::cout << n1 << ' ' << n2 << ' ' << n3 << ' ' << n4 << ' ' << n5 << '\n';
}
 
int g(int n1)
{
    return n1;
}
 
struct Foo {
    void print_sum(int n1, int n2)
    {
        std::cout << n1+n2 << '\n';
    }
    int data = 10;
};
 
int main()
{
    using namespace std::placeholders;  // for _1, _2, _3...
 
    // demonstrates argument reordering and pass-by-reference
    int n = 7;
    // (_1 and _2 are from std::placeholders, and represent future
    // arguments that will be passed to f1)
    auto f1 = std::bind(f, _2, _1, 42, std::cref(n), n);
    n = 10;
    f1(1, 2, 1001); // 1 is bound by _2, 2 is bound by _1, 1001 is unused
 
    // nested bind subexpressions share the placeholders
    auto f2 = std::bind(f, _3, std::bind(g, _3), _3, 4, 5);
    f2(10, 11, 12);
 
    // common use case: binding a RNG with a distribution
    std::default_random_engine e;
    std::uniform_int_distribution<> d(0, 10);
    std::function<int()> rnd = std::bind(d, e); // a copy of e is stored in rnd
    for(int n=0; n<10; ++n)
        std::cout << rnd() << ' ';
    std::cout << '\n';
 
    // bind to a member function
    Foo foo;
    auto f3 = std::bind(&Foo::print_sum, &foo, 95, _1);
    f3(5);
 
    // bind to member data
    auto f4 = std::bind(&Foo::data, _1);
    std::cout << f4(foo) << '\n';
 
    // smart pointers can be used to call members of the referenced objects, too
    std::cout << f4(std::make_shared<Foo>(foo)) << '\n'
              << f4(std::unique_ptr<Foo>(new Foo(foo))) << '\n';}

2 1 42 10 7
12 12 12 4 5
1 5 0 2 0 8 2 2 10 8
100
10
10
10

See also