CODEWITHSUNDEEP
c++operator-overloadinggcc4.8
Eric
Eric

Reputation: 97591

Why can I override the operators for - (negation) and ! (not) but not ~ (bitwise not)?

Why does the following define the ! and - operators, but fail to define the ~ operator?

#include <type_traits>
#include <cstdint>
#include <typeinfo>
#include <cstdio>

template <typename T, T v>
struct integral_constant
    : std::integral_constant<T, v>
{
};

#define DECL_UNARY_OP(op) \
    template <typename T, T t> \
    constexpr integral_constant<decltype(op t), (op t)> \
    operator op(integral_constant<T, t>) \
    { return {}; } \


DECL_UNARY_OP(~);
DECL_UNARY_OP(-);
DECL_UNARY_OP(!);

int main() {
    constexpr auto x = integral_constant<uint8_t, 1>{};
    constexpr auto y = integral_constant<uint8_t, 10>{};
    constexpr auto z = integral_constant<uint8_t, 100>{};
    puts(typeid(-x).name());  // integral_constant<...>
    puts(typeid(~y).name());  // int!
    puts(typeid(!z).name());  // integral_constant<...>
}

Compiling this on GCC-4.8.2 gives the following on godbolt, where you can clearly see that the middle operation has decayed away from an integral_constant type.

Why is this happening?

Upvotes: 1

Views: 133

Answers (1)

Eric
Eric

Reputation: 97591

A simpler reproduction can be obtained by eliminating std::integral_constant: 

#include <cstdint>
#include <typeinfo>
#include <cstdio>

template <typename T, T v>
struct integral_constant { };

template <typename T, T t>
constexpr integral_constant<decltype(~t), (~t)>
operator ~(integral_constant<T, t>) { return {}; }


int main() {
    constexpr auto y = integral_constant<uint8_t, 10>{};
    puts(typeid(~y).name());
}

Which now fails to compile with:

<source>: In function 'int main()':
<source>:17:17: error: no match for 'operator~' (operand type is 'const integral_constant<unsigned char, 10u>')
     puts(typeid(~y).name());
                 ^
<source>:17:17: note: candidate is:
<source>:11:1: note: template<class T, T t> constexpr integral_constant<decltype (~ t), (~ t)> operator~(integral_constant<T, t>)
 operator ~(integral_constant<T, t>)
 ^
<source>:11:1: note:   template argument deduction/substitution failed:
<source>: In substitution of 'template<class T, T t> constexpr integral_constant<decltype (~ t), (~ t)> operator~(integral_constant<T, t>) [with T = unsigned char; T t = 10u]':
<source>:17:18:   required from here
<source>:11:1: error: 't' was not declared in this scope
Compiler returned: 1

It seems this can be fixed by adding extra parentheses,

-integral_constant<decltype(~t), (~t)>
+integral_constant<decltype((~t)), (~t)>

Upvotes: 1

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