c++ - What is the value category of result yielded from shift operators, bit-wise operators, and sizeof operator? -

shift operators: << >> bit-wise operators: ~, &, ^, | sizeof operator: sizeof()

per c++ standard (n3797), can confirm ~ yields prvalue (5.3.1/2), not others above.

as far can tell results prvalues speculative. seems under-specified in similar way value categories of operands covered in what value category of operands of c++ operators when unspecified? , does standard mandate lvalue-to-rvalue conversion of pointer variable when applying indirection?.

we can see section 3.10 lvalues , rvalues has following note:

the discussion of each built-in operator in clause 5 indicates category of value yields , value categories of operands expects.

but can see section 5 spells out value category explicitly in few cases. in case of particular question spells out explicitly value category of result & , ~.

even though underspecified can find clues point in consistent direction. can make argument operands >>, <<, ^ , | converted prvalues. not dictate value category of result seem exclude result being xvalue per section 5 paragraph 7 has following note:

an expression xvalue if is:

— result of calling function, whether implicitly or explicitly, return type rvalue reference object type,

— cast rvalue reference object type,

— class member access expression designating non-static data member of non-reference type in object expression xvalue, or

— .* pointer-to-member expression in first operand xvalue , second operand pointer data member.

in general, effect of rule named rvalue references treated lvalues , unnamed rvalue references objects treated xvalues; rvalue references functions treated lvalues whether named or not.

i don't see reasonable argument result lvalue leaves prvalue.

so details follows:

both ~ , & covered section 5.3.1 unary operators paragraph 2 says:

the result of each of following unary operators prvalue.

the shift operators require integral promotions covered in section 5.8 shift operators paragraph 1 says:

the operands shall of integral or unscoped enumeration type , integral promotions performed.

and can see integral promotions require prvalues section 4.5 integral promotions says in every paragraph starts with:

a prvalue of [...]

both ^ , | require usual arithmetic conversions , both say:

operator applies integral or unscoped enumeration operands

and therefore last clause of usual arithmetic conversions applies says:

otherwise, integral promotions (4.5) shall performed on both operands.⁵⁹

empirical approach

luc danton has empirical approach determining value category of expression in answer empirically determine value category of c++11 expression?. approach uses following code:

template<typename t> struct value_category {     // or can integral or enum value     static constexpr auto value = "prvalue"; };  template<typename t> struct value_category<t&> {     static constexpr auto value = "lvalue"; };  template<typename t> struct value_category<t&&> {     static constexpr auto value = "xvalue"; };  // double parens ensuring inspect expression, // not entity #define value_category(expr) value_category<decltype((expr))>::value 

and answer outlines logic follows:

an lvalue expression results in lvalue reference type, xvalue in rvalue reference type, , prvalue in type.

the following examples yield prvalue (see live):

int x = 10, y = 2 ; int &xr = x ; int &yr = y ;  std::cout << value_category( x << y ) << std::endl ; std::cout << value_category( 10 << 2 ) << std::endl ; std::cout << value_category( xr << yr ) << std::endl ;  std::cout << value_category( x | y ) << std::endl ; std::cout << value_category( 10 | 2 ) << std::endl ;  std::cout << value_category( x ^ y ) << std::endl ; std::cout << value_category( 10 ^ 2 ) << std::endl ;  std::cout << value_category( sizeof( int ) ) << std::endl ; std::cout << value_category( sizeof( x ) ) << std::endl ;