Why does using "constexpr" give primary expression error in codeblocks

Question

I am getting expected primary expression before constexpr in codeblocks only. Is there a way to fix this? This is the code I am trying to test. I have set c++ 17 as the compiler. I am using GNU GCC Compiler. GCC (MinGW.org GCC-6.3.0-1) 6.3.0. I am not getting this error in Visual Studio.

#include <iostream>
#include <vector>

template<class T>
void testType(std::vector<T> &x)
{
    if constexpr (std::is_same_v<T, std::string>)
    {
        //push string
        std::cout<<"String\n";
    }
    else if constexpr (std::is_same_v<T,int>)
    {
        //push integer
        std::cout<<"int\n";
    }
}
int main()
{
    
    std::vector<std::string> x;
    
    testType(x);

    return 0;
}

This is the error I am getting.

error: expected primary-expression before 'constexpr'
error: expected ')' before 'constexpr'

After adding the missing `#include `, the shown code compiles without eny errors with gcc 10. gcc 6 is probably too old for full C++17 support. — Sam Varshavchik, Feb 04 '21 at 01:24
Works as of GCC 7.1. Sorry, chum, it's time to update. If you can update. Example: https://godbolt.org/z/qq86eE — user4581301, Feb 04 '21 at 01:30
it clearly complains about a syntax issue if statement in c++ `if (expression) {}` try with parenthesis. — Mehdi Karamosly, Aug 22 '23 at 13:59

score 2 · Accepted Answer · answered Feb 04 '21 at 02:22

Your version of GCC does not support if constexpr. Support was added in version 7.

You can look up which C++17 features are supported on the GCC website; the line relevant for this question is "constexpr if". There are a lot of "7"'s – and even one "8" – in the column for the first version that supports each feature, so you should expect C++17 support to be spotty with a compiler from the version 6 series (e.g. your 6.3.0).

Why does using "constexpr" give primary expression error in codeblocks

1 Answers1