c++ - loss of const volatile qualifiers -

i have compile error:

error c3848: expression having type 'const unicode::endian_swap<t>'              lose const-volatile qualifiers in order call              'unsigned long unicode::endian_swap<t>::operator ()(t &)' 

the description of error, here, doesn't explain what's going on.

i'm unable reproduce error in smaller sample, can show basic layout of 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 here     } }; 

the error popped after adding endian_swap member. actual class derived std::codecvt , installed std::locale.

can give better explanation of error 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! @ above template , see if can see too.

how instantiating template. instantiation of endian_swap uses type t, , pass unsigned short. unless t unsigned short, you'll need conversion, , results of conversion temporary, can't bind non-const reference.

you don't provide sscce, it's hard say. error message post refers call 'unsigned long unicode::endian_swap::operator ()(t &)'; either endian_swapis instantiated forunsigned long(in case, passing anunsigned short` require temporary), or code posted not code triggered error.