Книга: Standard Template Library Programmer

identity

identity<T>

Category: functors

Component type: type

Identity is a Unary Function that represents the identity function: it takes a single argument x, and returns x.

int main() {
 int x = 137;
 identity<int> id;
 assert(x == id(x));
}

Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h. This class is an SGI extension; it is not part of the C++ standard.

Adaptable Unary Function

None.

unary_function<T, T>

All of identity's members are defined in the Adaptable Unary Function requirements. Identity does not introduce any new members.

[1] It is essential that the return type and the argument type are the same: generalizing identity to allow them to differ would not work. The reason is that identity returns a const reference to its argument, rather than a copy of its argument. If identity were allowed to perform a conversion, then this would be a dangling reference.

The function object overview, select1st, select2nd, project1st, project2nd

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