Why isn't common_type<long, unsigned long>::type = long long?

Question

common_type<long, unsigned long>::type is unsigned long because concerning the operands after integral promotion the standard says...

[...] if the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type shall be converted to the type of the operand with unsigned integer type

Not to call the integral promotion system buggy, but it seems like if there is a bigger signed integer type which can can represent the range of both signed and unsigned operands it should be used.

I know some platforms might have long == long long, in which case the above rule can take effect. But if there is a larger signed integral type available, shouldn't it be used?

Answer 1

first of all, std::common_type (and of course boost::type_traits::common_type) use the ternary operator to retrieve the type result. In this case the relevant quote comes from the CppReference, 6b)

E2 and E3 have arithmetic or enumeration type: usual arithmetic conversions are applied to bring them to common type, that type is the result.

With this information we can find the rules for the usual arithmetic conversions in the c++ standard, 5p10, page 88.

— Otherwise, if the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type shall be converted to the type of the operand with unsigned integer type.

So basically the answer to your question is: ...because the standard says so.

But you are not the only one finding this behavior unexpected. Here's a quick runnable example to try:

#include <iostream>
#include <typeinfo>
#include <type_traits>

int main(int argc, const char* argv[])
{

    std::cout << typeid(std::common_type<char, unsigned char>::type).name() << std::endl;
    // I would expect "short", and the result is "int", ok so far.

    std::cout << typeid(std::common_type<short, unsigned short>::type).name() << std::endl;
    // I would expect "int", and the result is "int", yay.

    std::cout << typeid(std::common_type<int, unsigned int>::type).name() << std::endl;
    // I would expect "long", but the result is "unsigned int"

    std::cout << typeid(std::common_type<long, unsigned long>::type).name() << std::endl;
    // I would expect "long long", but the result is "unsigned long"


    // So this usual arithmetic conversion can lead to unexpected behavior:
    auto var_auto = true ? var_i : var_ui;
    std::cout << typeid(var_auto).name() << std::endl;   // unsigned int
    std::cout << var_auto << std::endl;                  // 4294967173

    return 0;
}

But that the current behavior is a problem is known, and a proposal exists to remove some of the surprises.

-Hannes