# 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`.

### See also

`identity`, `select2nd`, `project1st`, `project2nd`

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