# size

### Synopsis

    template<
          typename Sequence
        >
    struct size
    {
        typedef unspecified type;
    };

### Description

`size` returns the number of elements in the sequence, that is, the number of elements in the range `[begin<Sequence>::type,end<Sequence>::type)`.

### Definition

    #include "boost/mpl/size.hpp"

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | A model of Sequence |

### Expression semantics

Expression | Expression type | Precondition | Semantics | Postcondition
--- | --- | --- | --- | ---
`typedef size<Sequence>::type s;` | A model of Integral Constant |  | Equivalent to `typedef distance< begin<Sequence>::type,end<Sequence>::type >::type s;` | `s::value >= 0`

### Complexity

The complexity of the `size` algorithm directly depends on the implementation of the particular sequence it is applied to. In the worst case `size` has a linear complexity. As a general rule, if the `Sequence` is a Random Access Sequence, you can be certain that `size<Sequence>::type` is an amortized constant time operation. The opposite is not necessary true - for example, a model of Forward Sequence still can guarantee you an amortized constant time `size` complexity. Please refer the documentation page of the concrete sequence type for further information.

### Example

    typedef list0<> empty_list;
    typedef vector_c<int,0,1,2,3,4,5> numbers;
    typedef range_c<int,0,100> more_numbers;

    BOOST_STATIC_ASSERT(size<list>::type::value == 0);
    BOOST_STATIC_ASSERT(size<numbers>::type::value == 5);
    BOOST_STATIC_ASSERT(size<more_numbers>::type::value == 100);

### See also

Sequence, `empty`, `begin`, `end`

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Last edited July 17, 2002 3:51 am