20 General utilities library [utilities]

20.8 Function objects [function.objects]

20.8.9 Function template bind [bind]

20.8.9.1 Function object binders [func.bind]

1

This subclause describes a uniform mechanism for binding arguments of callable objects.

20.8.9.1.1 Class template is_bind_expression [func.bind.isbind]

namespace std {
  template<class T> struct is_bind_expression
    : integral_constant<bool, see below> { };
}
1

is_bind_expression can be used to detect function objects generated by bind. bind uses is_bind_expression to detect subexpressions. Users may specialize this template to indicate that a type should be treated as a subexpression in a bind call.

2

If T is a type returned from bind, is_bind_expression<T> shall be publicly derived from integral_constant<bool, true>, otherwise from integral_constant<bool, false>.

3

is_placeholder can be used to detect the standard placeholders _1, _2, and so on. bind uses is_placeholder to detect placeholders. Users may specialize this template to indicate a placeholder type.

4

If T is the type of std::placeholders::_J, is_placeholder<T> shall be publicly derived from integral_constant<int, J>, otherwise from integral_constant<int, 0>.

20.8.9.1.2 Function template bind [func.bind.bind]

1

In the text that follows, the following names have the following meanings:

  • (1.1)

    FD is the type decay<F>::type,

  • (1.2)

    fd is an lvalue of type FD constructed from std::forward<F>(f),

  • (1.3)

    Ti is the ith type in the template parameter back BoundArgs,

  • (1.4)

    TiD is the type decay<Ti>::type,

  • (1.5)

    ti is the ith argument in the function parameter pack bound_args,

  • (1.6)

    tid is an lvalue of type TiD constructed from std::forward<Ti>(ti),

  • (1.7)

    Uj is the jth deduced type of the UnBoundArgs&&... parameter of the forwarding call wrapper, and

  • (1.8)

    uj is the jth argument associated with Uj.

template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);

2

Requires: is_constructible<FD, F>::value shall be true. For each Ti in BoundArgs, is_constructible<TiD, Ti>::value shall be true. INVOKE (fd, w1, w2, ..., wN) ([func.require]) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args).

3

Returns: A forwarding call wrapper g with a weak result type ([func.require]). The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type), where cv represents the cv-qualifiers of g and the values and types of the bound arguments v1, v2, ..., vN are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of FD or of any of the types TiD throws an exception.

4

Throws: Nothing unless the construction of fd or of one of the values tid throws an exception.

5

Remarks: The return type shall satisfy the requirements of MoveConstructible. If all of FD and TiD satisfy the requirements of CopyConstructible, then the return type shall satisfy the requirements of CopyConstructible. [ Note: This implies that all of FD and TiD are MoveConstructible.  — end note ]

template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);

6

Requires: is_constructible<FD, F>::value shall be true. For each Ti in BoundArgs, is_constructible<TiD, Ti>::value shall be true. INVOKE(fd, w1, w2, ..., wN) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args).

7

Returns: A forwarding call wrapper g with a nested type result_type defined as a synonym for R. The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, v1, v2, ..., vN, R), where the values and types of the bound arguments v1, v2, ..., vN are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of FD or of any of the types TiD throws an exception.

8

Throws: Nothing unless the construction of fd or of one of the values tid throws an exception.

9

Remarks: The return type shall satisfy the requirements of MoveConstructible. If all of FD and TiD satisfy the requirements of CopyConstructible, then the return type shall satisfy the requirements of CopyConstructible. [ Note: This implies that all of FD and TiD are MoveConstructible.  — end note ]

10

The values of the bound arguments v1, v2, ..., vN and their corresponding types V1, V2, ..., VN depend on the types TiD derived from the call to bind and the cv-qualifiers cv of the call wrapper g as follows:

  • (10.1)

    if TiD is reference_wrapper<T>, the argument is tid.get() and its type Vi is T&;

  • (10.2)

    if the value of is_bind_expression<TiD>::value is true, the argument is tid(std::forward<Uj>(uj)...) and its type Vi is result_of<TiD cv (Uj...)>::type;

  • (10.3)

    if the value j of is_placeholder<TiD>::value is not zero, the argument is std::forward<Uj>(uj) and its type Vi is Uj&&;

  • (10.4)

    otherwise, the value is tid and its type Vi is TiD cv &.

20.8.9.1.3 Placeholders [func.bind.place]

namespace std {
  namespace placeholders {
    // M is the implementation-defined number of placeholders
    extern unspecified _1;
    extern unspecified _2;
                .
                .
                .
    extern unspecified _M;
  }
}
1

All placeholder types shall be DefaultConstructible and CopyConstructible, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types are CopyAssignable. CopyAssignable placeholders' copy assignment operators shall not throw exceptions.