This subclause describes a uniform mechanism for binding arguments of callable objects.
namespace std {
template<class T> struct is_bind_expression; // see below
}
is_bind_expression can be used to detect function objects generated by bind. bind uses is_bind_expression to detect subexpressions.
Instantiations of the is_bind_expression template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of true_type if T is a type returned from bind, otherwise it shall have a BaseCharacteristic of false_type. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of true_type to indicate that T should be treated as a subexpression in a bind call.
namespace std {
template<class T> struct is_placeholder; // see below
}
is_placeholder can be used to detect the standard placeholders _1, _2, and so on. bind uses is_placeholder to detect placeholders.
Instantiations of the is_placeholder template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has the BaseCharacteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it shall have a BaseCharacteristic of integral_constant<int, 0>. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.
In the text that follows:
FD is the type decay_t<F>,
fd is an lvalue of type FD constructed from std::forward<F>(f),
Ti is the ith type in the template parameter pack BoundArgs,
TiD is the type decay_t<Ti>,
ti is the ith argument in the function parameter pack bound_args,
tid is an lvalue of type TiD constructed from std::forward<Ti>(ti),
Uj is the jth deduced type of the UnBoundArgs&&... parameter of the forwarding call wrapper, and
uj is the jth argument associated with Uj.
template<class F, class... BoundArgs>
unspecified bind(F&& f, BoundArgs&&... bound_args);
Requires: is_constructible_v<FD, F> shall be true. For each Ti in BoundArgs, is_constructible_v<TiD, Ti> 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). The cv-qualifiers cv of the call wrapper g, as specified below, shall be neither volatile nor const volatile.
Returns: A forwarding call wrapper g ([func.require]). The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN)), 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.
Throws: Nothing unless the construction of fd or of one of the values tid throws an exception.
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);
Requires: is_constructible_v<FD, F> shall be true. For each Ti in BoundArgs, is_constructible_v<TiD, Ti> shall be true. INVOKE(fd, w1, w2, ..., wN) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args). The cv-qualifiers cv of the call wrapper g, as specified below, shall be neither volatile nor const volatile.
Returns: A forwarding call wrapper g ([func.require]). The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(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.
Throws: Nothing unless the construction of fd or of one of the values tid throws an exception.
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 ]
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:
if TiD is reference_wrapper<T>, the argument is tid.get() and its type Vi is T&;
if the value of is_bind_expression_v<TiD> is true, the argument is tid(std::forward<Uj>(uj)...) and its type Vi is result_of_t<TiD cv & (Uj&&...)>&&;
if the value j of is_placeholder_v<TiD> is not zero, the argument is std::forward<Uj>(uj) and its type Vi is Uj&&;
otherwise, the value is tid and its type Vi is TiD cv &.
namespace std::placeholders {
// M is the implementation-defined number of placeholders
see below _1;
see below _2;
.
.
.
see below _M;
}
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.
Placeholders should be defined as:
constexpr unspecified _1{};
If they are not, they shall be declared as:
extern unspecified _1;