The MPL Reference Manual: value_type

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# value_type

### Synopsis

    template<
          typename Sequence
        , typename X
        >
    struct value_type
    {
        typedef unspecified type;
    };

### Description

Returns the value that would be used for element `X` in `Sequence`.

### Header

    #include <boost/mpl/value_type.hpp>

### Model of

Tag Dispatched Metafunction

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Associative Sequence | A sequence to query.
`X` | Any type | The type to get the value for.

### Expression semantics

For any Associative Sequence `s`, and an artibrary type `x`:

    typedef value_type<s,x>::type v;

Return type: | A type.
Precondition: | `x` can be put in `s`.
Semantics: | `v` is the value that would be used for `x` in `s`.
Postcondition: | If .. parsed-literal:
    has_key< s,key_type<s,x>::type >::type
then .. parsed-literal:
    at< s,key_type<s,x>::type >::type
is identical to `value_type<s,x>::type`.

### Complexity

Amortized constant time.

### Example

    typedef value_type< map<>,pair<int,unsigned> >::type v1;
    typedef value_type< set<>,pair<int,unsigned> >::type v2;

    BOOST_MPL_ASSERT(( is_same< v1,unsigned > ));
    BOOST_MPL_ASSERT(( is_same< v2,pair<int,unsigned> > ));

### See also

Associative Sequence, key_type, at, set, map