Why SFINAE doesnt work here

Asked Jan 16 '13 at 10:05

Active Dec 29 '17 at 01:42

Viewed 176 times

I want to implement static check to find out if type constructible:

#include <iostream>

template<typename T>
struct IsConstr
{
    typedef char TrueType;
    typedef struct{char a[2];} FalseType;
    template<typename C>
    static decltype((C(),TrueType())) test(int);
    template<typename C>
    static FalseType test(...);

    enum{value=(sizeof(test<T>(0))==sizeof(TrueType))};
}; 


struct S{private: S();};
int main()
{
    std::cout<<IsConstr<S>::value<<std::endl;
    std::cout<<IsConstr<int>::value<<std::endl;
    return 0;
}

The code doesn't compile, showing an error

"Substitution failed. 'S::S()' is private".

Why doesn't SFINAE work here? Thanks.

edited Dec 29 '17 at 01:42

O'Neil

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asked Jan 16 '13 at 10:05

Alexander

You might be intrested in reading http://stackoverflow.com/questions/8984013/can-sfinae-detect-private-access-violations/8984379#8984379 (btw. clang likes your code) – PlasmaHH Jan 16 '13 at 10:10
It compiles with clang 3.0-6ubuntu3, it does not with g++ 4.7.2-2ubuntu1 – zch Jan 16 '13 at 10:14
Weren't some SFINAE functionality disabled by C++11? – Bartek Banachewicz Jan 16 '13 at 10:21
2

It's a bug, g++ doesn't compile even `std::is_default_constructible`. – zch Jan 16 '13 at 10:22
Good to know that bug is in gcc, not in my code. Thanks. – Alexander Jan 16 '13 at 11:02
Note that this also compiles with gcc from svn – PlasmaHH Jan 16 '13 at 16:42
It works in modern compilers: https://gcc.godbolt.org/z/1fWxn64qM – Fedor Dec 18 '21 at 05:31

Why SFINAE doesnt work here

0 Answers0