混合模板类和模板方法时出现C ++错误

Question

I have a class that takes a size type as a parameter in case one doesn't want to use size_t or another hard coded type which would require casting and checking for overflows. This class also has some methods taking a second template type :

template< typename SizeType = uint32_t >
class BufferReader
{
    SizeType m_nPosition;

    template< typename T >
    T Read();
}

My problem is, what is the syntax to implement this ?

template< typename SizeType, typename T >
T BufferReader< SizeType >::Read()
{
    // ...
}

Which gives me an error :

invalid use of incomplete type 'class core::BufferReader<SizeType>'

Or :

template< typename T >
template< typename SizeType >
T BufferReader< SizeType >::Read()
{
    // ...
}

Which gives the same error.

Or anything else ?

Thank you !

Answer 1

Your second example is nearly right, you just need to have the template parameters of the class first and those of the function second:

template< typename SizeType >
template< typename T >
T BufferReader< SizeType >::Read()
{
    // ...
}

Answer 2

You almost got it. You have to stack the template s as in the declaration: first the class and then the member.

template < typename SizeT >
template < typename T >
T
BufferReader<SizeT>::Read()
{
  return T {};
}

Note that the code can be simplified by defining the function right in the class body but I understand that some people will prefer to separate declaration from definition for aesthetic reasons.