CODEWITHSUNDEEP
c++visual-c++
Septimiu Neaga
Septimiu Neaga

Reputation: 121

std::enable_if not working properly in MSVC compiler

I have following code snippet trying to understand std::enable_if.

Using godbolt I noticed that I worked on almost every compiler (gcc 6.3.0 and onwards) but it does not compile on any version of MSVC (tested on msvc 2015, 2017, 2019).

#include <cstdint>
#include <iostream>

template <typename T, typename std::enable_if<(std::is_arithmetic<T>::value), bool>::type = true>
constexpr std::size_t bitSize()
{
    return sizeof(T) * 8;
}

template <typename T, typename std::enable_if <(bitSize<T>() == 8), bool>::type = true>
void f(T val) {
    std::cout << "f val = " << val << std::endl;
}

int main()
{
    uint8_t u8 = 1;
    f<uint8_t>(u8);
    
    return 1;
}

MSVC compiler returns:

main.cpp(10,49): error C2672: 'bitSize': no matching overloaded function found
main.cpp(5,23): message : could be 'size_t bitSize(void)'
main.cpp(10,60): message : 'size_t bitSize(void)': could not deduce template argument for '__formal'
main.cpp(18,2): error C2672: 'f': no matching overloaded function found
main.cpp(11,6): message : could be 'void f(T)'
main.cpp(18,15): message : 'void f(T)': could not deduce template argument for '__formal'

Any idea if this is a known issue or am I missing something

Removing std::enable_if from bitSize resolves the issue so I assume it has something to do with nested std::enable_if

This code works as example:

#include <cstdint>
#include <iostream>

template <typename T>
constexpr std::size_t bitSize()
{
    return sizeof(T) * 8;
}

template <typename T, typename std::enable_if <(bitSize<T>() == 8), bool>::type = true>
void f(T val) {
    std::cout << "f val = " << val << std::endl;
}

int main()
{
    uint8_t u8 = 1;
    f<uint8_t>(u8);
    
    return 1;
}

Upvotes: 9

Views: 358

Answers (1)

Septimiu Neaga
Septimiu Neaga

Reputation: 121

I understand that MSVC has problems deducing template parameters with std::enable_if.

In the end, the solution that looks simple and clean would be:

#include <cstdint>
#include <iostream>

template <typename T>
constexpr typename std::enable_if<(std::is_arithmetic<T>::value), std::size_t>::type bitSize()
{
    return sizeof(T) * 8;
}

template <typename T>
typename std::enable_if <(bitSize<T>() == 8)>::type f(T val) {
    std::cout << "f val = " << val << std::endl;
}

int main()
{
    uint8_t u8 = 'a';
    f<uint8_t>(u8);
    
    return 1;
}

https://godbolt.org/z/W83bYddMT

This solution serves the same purpose, works on other compilers and avoids adding 'true' as a second parameter.

Upvotes: 3

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