std::bind(3)

NAME
std::bind - std::bind

Synopsis
   Defined in header <functional>
   template< class F, class... Args > (since C++11)
   /* unspecified */ bind( F&& f, Args&&... args ); (until C++20)
   template< class F, class... Args >
   constexpr /* unspecified */ bind( F&& f, (since C++20)
   Args&&... args ); (1)
   template< class R, class F, class... Args > (since C++11)
   /* unspecified */ bind( F&& f, Args&&... args ); (until C++20)
   template< class R, class F, class... Args > (2)
   constexpr /* unspecified */ bind( F&& f, (since C++20)
   Args&&... args );
   The function template std::bind generates a forwarding call wrapper for f. Calling
   this wrapper is equivalent to invoking f with some of its arguments bound to args.
   If std::is_constructible<std::decay<F>::type, F>::value is false, or
   std::is_constructible<std::decay<Arg_i>::type, Arg_i>::value is false for any type
   Arg_i in Args, the program is ill-formed.
   If std::decay<Ti>::type or any type in Args is not MoveConstructible or
   Destructible, the behavior is undefined.

Parameters
          Callable object (function object, pointer to function, reference to function,
   f - 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

Return value
   A function object g of unspecified type T, for which
   std::is_bind_expression<T>::value is 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) (deprecated in C++17) 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 (until C++20)
   type with nested typedef result_type, then result_type is
   F::result_type. Otherwise no result_type is defined.
   2) (deprecated in C++17) result_type is exactly R.

Member function operator()
   When g is invoked in a function call expression g(u1, u2, ... uM), an invocation of
   the stored object takes place, as if by
   1) INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ...,
   std::forward<VN>(vN)), or
   2) INVOKE<R>(fd, std::forward<V1>(v1), std::forward<V2>(v2), ...,
   std::forward<VN>(vN)),
   where fd is a value of type std::decay<F>::type, the values and types of the bound
   arguments v1, v2, ..., vN are determined as specified below.
   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.
   An invocation of operator() is non-throwing
   or is a constant subexpression
   (since C++20) if and only if so is the underlying INVOKE operation. operator()
   participates in overload resolution only if the INVOKE operation is well-formed when
   treated as an unevaluated operand.
   If g is volatile-qualified, the program is ill-formed.
   If INVOKE(fd, w1, w2, ..., wN) can never be a valid expression for any possible
   values w1, w2, ..., wN, the behavior is undefined.
   Bound arguments
   For each stored argument arg_i, the corresponding bound argument v_i in the INVOKE
   or INVOKE<R> operation is determined as follows:
     Case 1: reference wrappers
   If arg_i is of type std::reference_wrapper<T> (for example, std::ref or std::cref
   was used in the initial call to std::bind), then v_i is arg_i.get() and its type V_i
   is T&: the stored argument is passed by reference into the invoked function object.
     Case 2: bind expressions
   If arg_i is of type T for which std::is_bind_expression<T>::value is true (for
   example, another std::bind expression was passed directly into the initial call to
   std::bind), then std::bind performs function composition: instead of passing the
   function object that the bind subexpression would return, the subexpression is
   invoked eagerly, and its return value is passed to the outer invokable object. If
   the bind subexpression has any placeholder arguments, they are shared with the outer
   bind (picked out of u1, u2, ...). Specifically, v_i is
   arg_i(std::forward<Uj>(uj)...) and its type V_i is
   std::result_of<T cv &(Uj&&...)>::type&&
   (until C++17)
   std::invoke_result_t<T cv &, Uj&&...>&&
   (since C++17) (cv-qualification is the same as that of g).
     Case 3: placeholders
   If arg_i is of type T, for which std::is_placeholder<T>::value is not 0 (meaning, a
   placeholder such as std::placeholders::_1, _2, _3, ... was used as the argument to
   the initial call to std::bind), then the argument indicated by the placeholder (u1
   for _1, u2 for _2, etc) is passed to the invokable object: v_i is
   std::forward<Uj>(uj) and its type V_i is Uj&&.
     Case 4: ordinary arguments
   Otherwise, arg_i is passed to the invokable object as lvalue argument: v_i is simply
   arg_i and its type V_i is T cv &, where cv is the same cv-qualification as that of
   g.

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
// Run this code
 #include <functional>
 #include <iostream>
 #include <memory>
 #include <random>
 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...
     std::cout << "1) 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, 42, _1, std::cref(n), n);
     n = 10;
     f1(1, 2, 1001); // 1 is bound by _1, 2 is bound by _2, 1001 is unused
                     // makes a call to f(2, 42, 1, n, 7)
     std::cout << "2) achieving the same effect using a lambda: ";
     n = 7;
     auto lambda = [&ncref = n, n](auto a, auto b, auto /*unused*/)
     {
         f(b, 42, a, ncref, n);
     };
     n = 10;
     lambda(1, 2, 1001); // same as a call to f1(1, 2, 1001)
     std::cout << "3) nested bind subexpressions share the placeholders: ";
     auto f2 = std::bind(f, _3, std::bind(g, _3), _3, 4, 5);
     f2(10, 11, 12); // makes a call to f(12, g(12), 12, 4, 5);
     std::cout << "4) bind a RNG with a distribution: ";
     std::default_random_engine e;
     std::uniform_int_distribution<> d(0, 10);
     auto 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';
     std::cout << "5) bind to a pointer to member function: ";
     Foo foo;
     auto f3 = std::bind(&Foo::print_sum, &foo, 95, _1);
     f3(5);
     std::cout << "6) bind to a mem_fn that is a pointer to member function: ";
     auto ptr_to_print_sum = std::mem_fn(&Foo::print_sum);
     auto f4 = std::bind(ptr_to_print_sum, &foo, 95, _1);
     f4(5);
     std::cout << "7) bind to a pointer to data member: ";
     auto f5 = std::bind(&Foo::data, _1);
     std::cout << f5(foo) << '\n';
     std::cout << "8) bind to a mem_fn that is a pointer to data member: ";
     auto ptr_to_data = std::mem_fn(&Foo::data);
     auto f6 = std::bind(ptr_to_data, _1);
     std::cout << f6(foo) << '\n';
     std::cout << "9) use smart pointers to call members of the referenced objects: ";
     std::cout << f6(std::make_shared<Foo>(foo)) << ' '
               << f6(std::make_unique<Foo>(foo)) << '\n';
 }

Output:
 1) argument reordering and pass-by-reference: 2 42 1 10 7
 2) achieving the same effect using a lambda: 2 42 1 10 7
 3) nested bind subexpressions share the placeholders: 12 12 12 4 5
 4) bind a RNG with a distribution: 0 1 8 5 5 2 0 7 7 10
 5) bind to a pointer to member function: 100
 6) bind to a mem_fn that is a pointer to member function: 100
 7) bind to a pointer to data member: 10
 8) bind to a mem_fn that is a pointer to data member: 10
 9) use smart pointers to call members of the referenced objects: 10 10
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       1. the bounded arguments
                        were not forwarded to fd 1. forwarded
   LWG 2021 C++11 2. in case 2, the type of V_i 2. changed to
                       was std::result_of<T
                        std::result_of<T cv &(Uj&&...)>::type&&
                       cv (Uj...)>::type

See also
   bind_front bind a variable number of arguments, in order, to a function
   bind_back object
   (C++20) (function template)
   (C++23)
   _1, _2, _3, _4, ... placeholders for the unbound arguments in a std::bind expression
   (C++11) (constant)
   mem_fn creates a function object out of a pointer to a member
   (C++11) (function template)