c++ - Force a returned const value to move not copy -

i know ok:

struct foo {     foo& operator=( foo& ) = delete; // disallow assign     foo( int ) { }                   // basic constructor     foo( const foo& ) = delete;      // disallow copy     foo( foo&& ) { }                 // allow move };  foo getfoo( void ) {     return foo( 3 ); } foo foo = getfoo( ); // uses move constructor 

whereas use copy constructor, , therefore invalid foo (and object with copy constructor, valid inefficient):

const foo getconstfoo( void ) {     return foo( 3 ); } const foo constfoo = getconstfoo( ); // error: copy constructor deleted! 

but ok: (and extend life of constfooref)

const foo& constfooref = getconstfoo( ); // uses move constructor 

now, second case call

foo( const foo&& ) { } 

if it's available. know if there's way detect final object const inside constructor. if there is, can apply move semantics (since both involved objects const , parameter isn't being used elsewhere it's ok cast away const-ness), , make second case legal.

so while question title explains end-result i'm trying achieve, sub-problem detecting whether object const after has been constructed (naïvely tried foo( const foo&& ) const { }, no luck there!)

you might do

foo::constant getconstfoo( void ) {     return foo::constant( 3 ); } 

where foo::constant immutable

struct foo {     struct constant     {        constant(foo&&);     };     foo& operator=( foo& ) = delete; // disallow assign     foo( int ) { }                   // basic constructor     foo( const foo& ) = delete;      // disallow copy     // might not use: foo( foo&& ) { }                 // allow move };