C++ std::enable_if differentiate between floating point and signed number?

Question

How can I make the following differentiate between signed numbers and floating point numbers?

template<class T, typename std::enable_if<std::is_signed<T>::value>::type* = nullptr >
const char* GetTypeName(T t)
{
    if (std::numeric_limits<T>::max() == std::numeric_limits<signed char>::max())
        return "int8";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<signed short>::max())
        return "int16";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<signed int>::max())
        return "int32";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<signed long long>::max())
        return "int64";
    else
        return nullptr;
}

template<class T, typename std::enable_if<std::is_unsigned<T>::value>::type* = nullptr >
const char* GetTypeName(T t)
{
    if (std::numeric_limits<T>::max() == std::numeric_limits<unsigned char>::max())
        return "uint8";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<unsigned short>::max())
        return "uint16";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<unsigned int>::max())
        return "uint32";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<unsigned long long>::max())
        return "uint64";
    else
        return nullptr;
}

template<class T, typename std::enable_if<std::is_floating_point<T>::value>::type* = nullptr >
const char* GetTypeName(T t)
{
    if (std::numeric_limits<T>::max() == std::numeric_limits<Float32>::max())
        return "float";
    else if (std::numeric_limits<T>::max() == std::numeric_limits<Float64>::max())
        return "double";
    else
        return nullptr;
}

Works perfectly with integral types but it cannot differentiate between signed numbers and floating point numbers:

// Works as expected
std::string tsig = GetTypeName<signed char>(static_cast<signed char>(0));
std::string tsig = GetTypeName<signed short>(static_cast<signed short>(0));
std::string tsig = GetTypeName<signed int>(static_cast<signed int>(0));
std::string tsig = GetTypeName<signed long long>(static_cast<signed long long>(0));

// Works as expected
std::string tsig = GetTypeName<unsigned char>(static_cast<unsigned char>(0));
std::string tsig = GetTypeName<unsigned short>(static_cast<unsigned short>(0));
std::string tsig = GetTypeName<unsigned int>(static_cast<unsigned int>(0));
std::string tsig = GetTypeName<unsigned long long>(static_cast<unsigned long long>(0));

// This is ambiguous!
std::string tsig = GetTypeName<float>(static_cast<float>(0));
std::string tsig = GetTypeName<double>(static_cast<double>(0));

Answer 1

Use the std::is_floating_point trait along with std::is_signed.

template<class T, typename std::enable_if<std::is_floating_point<T>::value>::type* = nullptr >
const char* GetTypeName(T t)
{
  // called for floating point types
}

template<class T, typename std::enable_if<std::is_signed<T>::value &&
                                          !std::is_floating_point<T>::value>::type* = nullptr >
const char* GetTypeName(T t)
{
  // called for signed integral types
}

---

Your example can be reimplemented using overload resolution too, which avoids all the if-else ifs and numeric_limits verbosity.

template<typename T>
const char *GetTypeName(T) { return "<unknown>"; }

const char *GetTypeName(signed char) { return "int8"; }
const char *GetTypeName(signed short) { return "int16"; }
const char *GetTypeName(signed int) { return "int32"; }
const char *GetTypeName(signed long) { return "int64"; }

std::cout << GetTypeName(static_cast<signed char>(0)) << std::endl;
std::cout << GetTypeName(static_cast<signed short>(0)) << std::endl;
std::cout << GetTypeName(static_cast<signed int>(0)) << std::endl;
std::cout << GetTypeName(static_cast<signed long>(0)) << std::endl;
std::cout << GetTypeName(static_cast<signed long long>(0)) << std::endl;

Live demo