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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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select1st<Pair>


Category: functors	Component type: type

Description

Select1st is a function object that takes a single argument, a pair [1], and returns the pair's first element.

Example

Print all of a map's keys.

int main()
{
  map<int, double> M;
  M[1] = 0.3;
  M[47] = 0.8;
  M[33] = 0.1;

  transform(M.begin(), M.end(), ostream_iterator<int>(cout, " "),
            select1st<map<int, double>::value_type>());
  // The output is  1 33 47.
}

Definition

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

Template parameters

Parameter	Description	Default
`Pair`	The function object's argument type.

Model of

Adaptable Unary Function Adaptable Unary Function Adaptable Unary Function

Type requirements

There exist some types U and V such that Pair provides the same interface as a pair<U,V>. [1]

Public base classes

unary_function<Pair, Pair::first_type>

Members

Member	Where defined	Description
`argument_type`	Adaptable Unary Function	The type of `select1st`'s argument: `Pair`
`result_type`	Adaptable Unary Function	The type of the result: `Pair::first_type`
const Pair::first_type& operator()(const Pair& p) const	Adaptable Unary Function	Function call. The return value is `p.first`.

New members

All of select1st's members are defined in the Adaptable Unary Function requirements. Select1st does not introduce any new members.

Notes

[1] Pair is not actually required to be a pair<U,V>, but merely to support the same interface as pair. In almost all cases the template parameter will be a pair, but it is occasionally useful for it to be something else. One example is a struct that has the members first, second, and third.