bslmf_removeextent

Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage

Purpose

Provide a metafunction to return an array type's element type.

Classes

• bsl::remove_extent: type trait that returns the element type of an array
• bsl::remove_extent_t: alias to the return type of the bsl::remove_extent

See also

bslmf_decay

Description

This component provides a metafunction bsl::remove_extent that returns the element type of an array. The functionality is intended to be identical to the C++11 metafunction std::remove_extent. From the C++14 standard:

If T names a type "array of `U`", the member typedef type shall be U, otherwise T. [ Note: For multidimensional arrays, only the first array dimension is removed. For a type "array of `const U`", the resulting type is const U. – end note ]

Usage

The class template Traverser is used to traverse an array and perform some operation. In order to do its job in the case of two-dimensional arrays, Traverser must hold on to an entire row of the array at a time in order to process it correctly. The row type is determined from the array type using remove_extent :

template <class ARRAY_TYPE>
class Traverser {
  public:
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES

Note that if the current compiler supports alias templates C++11 feature, we can use bsl::remove_extent_t alias to the "result" type of the bsl::remove_extent meta-function, that avoids the ::type suffix and typename prefix in the declaration of the function return type:

    using RowType = bsl::remove_extent_t<ARRAY_TYPE>;
#else
    typedef typename bsl::remove_extent<ARRAY_TYPE>::type RowType;
#endif
 
  private:
    RowType d_row;  // Might be scalar
    // ...
 
    // CREATORS
    Traverser() : d_row() {}
};

Now we can see that the row type is the type of the array after having striped off the high-order dimension:

int main()
{
    assert((bsl::is_same<int, Traverser<int>::RowType>::value));
    assert((bsl::is_same<int, Traverser<int[]>::RowType>::value));
    assert((bsl::is_same<int, Traverser<int[5]>::RowType>::value));
    typedef const int MyRow[6];
    assert((bsl::is_same<MyRow, Traverser<MyRow[]>::RowType>::value));
    assert((bsl::is_same<int[6], Traverser<int[7][6]>::RowType>::value));
 
    return 0;
}