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# std::bind

From cppreference.com
< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
template< class F, class... Args > /*unspecified*/ bind( F&& f, Args&&... args ); | (1) | (since C++11)
template< class R, class F, class... Args > /*unspecified*/ bind( F&& f, Args&&... args ); | (2) | (since C++11)

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`.

### [edit] Parameters

f | - | `Callable` object (function object, pointer to function, reference to function, pointer to member function, or pointer to data member) that will be bound to some arguments
args | - | list of arguments to bind, with the unbound arguments replaced by the placeholders `_1, _2, _3...` of namespace `std::placeholders`

### [edit] Return value

A function object of unspecified type `T`, for which std::is_bind_expression<T>::value == true, and which can be stored in std::function. 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 (i.e. another `bind` subexpression was used as an argument in the initial call to `bind`), then that bind subexpression is invoked immediately 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 (i.e., `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-is.

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.

### [edit] 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`.

### [edit] 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.

### [edit] Example

Run this code

    #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 _1, 2 is bound by _2, 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';}

Output:

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

### [edit] See also

_1, _2, _3, _4, ... (C++11) | placeholders for the unbound arguments in a `std::bind` expression (constant) [edit]
mem_fn (C++11) | creates a function object out of a pointer to a member (function template) [edit]

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