如何将const限定符添加到vector <> :: pointer？

Question

I'm trying to pass a const pointer to an element of a std::vector to a function, but I can't seem to get the function's signature right. I must be missing something trivial here, but I'm confused.

This is the minimal example that reproduces the issue:

#include <vector>
#include <functional>

class Image { void* ptr; };

using ImageConstRefArray = std::vector< std::reference_wrapper< Image const >>;

template< typename T = void, typename... OtherTs >
void TestDataType( const ImageConstRefArray::pointer images ) {
   // stuff.
   TestDataType< OtherTs... >( images + 1 );
}
template<>
inline void TestDataType<>( const ImageConstRefArray::pointer /*images*/ ) {} // End of iteration

template< typename... Types >
void Function( ImageConstRefArray const& images ) {
   TestDataType< Types... >( images.data() );
}

int main() {
   Image img1, img2;
   ImageConstRefArray array{ img1, img2 };
   Function( array );
}

This is GCC's (5.4) error message:

test.cpp: In instantiation of ‘void Function(const ImageConstRefArray&) [with Types = {}; ImageConstRefArray = std::vector<std::reference_wrapper<const Image> >]’:
test.cpp:24:20:   required from here
test.cpp:18:28: error: no matching function for call to ‘TestDataType(const std::reference_wrapper<const Image>*)’
    TestDataType< Types... >( images.data() );
                            ^
test.cpp:9:6: note: candidate: template<class T, class ... OtherTs> void TestDataType(std::vector<std::reference_wrapper<const Image> >::pointer)
 void TestDataType( const ImageConstRefArray::pointer images ) {
      ^
test.cpp:9:6: note:   template argument deduction/substitution failed:
test.cpp:18:41: note:   cannot convert ‘(& images)->std::vector<_Tp, _Alloc>::data<std::reference_wrapper<const Image>, std::allocator<std::reference_wrapper<const Image> > >()’ (type ‘const std::reference_wrapper<const Image>*’) to type ‘std::vector<std::reference_wrapper<const Image> >::pointer {aka std::reference_wrapper<const Image>*}’
    TestDataType< Types... >( images.data() );

So basically it's trying to put a const std::reference_wrapper<const Image>* into a std::reference_wrapper<const Image>* . The function's signature has const ImageConstRefArray::pointer as the parameter. If that const doesn't make the pointer a const pointer, then how do I write the function signature? Is the only solution to write out const std::reference_wrapper<const Image>* ? That solves the issue, but I'd rather write it in terms of ImageConstRefArray .

Answer 1

For const ImageConstRefArray::pointer , const is qualifed on the pointer itself, so it'll be std::reference_wrapper<const Image>* const ( const pointer to non-const), but not std::reference_wrapper<const Image> const * (non-const pointer to const ). (Note the different position of const .)

You should use std::vector::const_pointer instead, which will give you the type of pointer to const T . eg

template< typename T = void, typename... OtherTs >
void TestDataType( ImageConstRefArray::const_pointer images ) {
   // stuff.
   TestDataType< OtherTs... >( images + 1 );
}
template<>
inline void TestDataType<>( ImageConstRefArray::const_pointer /*images*/ ) {} // End of iteration