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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) }; }; Upda

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