loss of const volatile qualifiers

Question

I have a compile error:

error C3848: expression having type 'const unicode::endian_swap<T>'
             would lose some const-volatile qualifiers in order to call
             'unsigned long unicode::endian_swap<T>::operator ()(T &)'

The description of this error, Here, doesn't really explain what's going on.

I'm unable to reproduce the error in a smaller sample, but I can show the basic layout of my class.

template < typename T >
struct endian_swap
{
    endian_swap ( void ) {}
    T operator () ( T& _val ) const { return _val >> 8 | _val << 8; }
};

template < typename T >
struct test
{
    endian_swap< T > _swap;

    virtual void do_it ( ) const
    {
        unsigned short n = 0x1234;
        unsigned short * _dest = &n;

        *_dest++ = _swap( n ); // <-- Error is here
    }
};

The error popped up after adding endian_swap as a member. The actual class is derived from std::codecvt and installed into an std::locale.

Can somebody give a better explanation of the error than the site above.

Edit: Actual code:

template < typename T, size_t N = sizeof( T ) > struct endian_swap
{
    endian_swap ( void ) {};
    T operator () ( const T _val ) const { return _val };
};
template < typename T > struct endian_swap< T, 2 >
{
    endian_swap ( void ) {}
    T operator () ( const T _val ) const { return _val >> 8 | _val << 8; }
};
template < typename T > struct endian_swap< T, 4 >
{
    endian_swap ( void ) {};
    T operator () ( const T _val ) const { return (_val >> 24) | ((_val & 0x00ff0000) >> 8) | ((_val & 0x0000ff00) << 8) | (_val << 24) };
};

Update: Found it! Look carefully at the above template and see if you can see it too.

Answer 1

How are you instantiating the template. Your instantiation of endian_swap uses type T, and you pass it an unsigned short. Unless T is unsigned short, you'll need a conversion, and the results of a conversion are a temporary, which can't bind to a non-const reference.

You don't provide a SSCCE, so it's hard to say. But the error message you post refers to a call to unsigned long unicode::endian_swap<T>::operator ()(T &); either endian_swap is instantiated for unsigned long (in which case, passing an unsigned short would require a temporary), or the code you posted is not the code which triggered the error.