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Standard Template Library Programmer's Guide
(document number: 007-3426-004 / published: 1999-05-21) table of contents | additional info | download
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identity<T>


Category: functors	Component type: type

Description

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

Example

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

Definition

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.

Template parameters

Parameter	Description	Default
`T`	The function object's argument type, and return type. [1]

Model of

Adaptable Unary Function

Type requirements

None.

Public base classes

unary_function<T, T>

Members

Member	Where defined	Description
`argument_type`	Adaptable Unary Function	The type of `identity`'s argument: `T`.
`result_type`	Adaptable Unary Function	The type of the result: `T`. [1]
`const T& operator()(const T&) const`	Adaptable Unary Function	Function call. The return value is simply the argument.

New members

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

Notes

[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.