CODEWITHSUNDEEP
c++visual-studio-2012
graywolf
graywolf

Reputation: 7500

How to fail static_assert on function use

I want compiler to complain when I try to print instances of one class, let's say T2. In VS2013+ I can use:

template <typename T = float>
std::ostream & operator<<(std::ostream & os, const T2 & t2) {
    static_assert(std::is_integral<T>::value, "Fail in << for T2");
    return os;
}

however, that does not work in VS2012 (error C4519: default template arguments are only allowed on a class template). Any ideas how to achieve this in VS2012?

Upvotes: 1

Views: 93

Answers (2)

Zebrafish
Zebrafish

Reputation: 13876

"I want compiler to complain when I try to print instances of one class, let's say T2"

#include <iostream>

struct T1 { int a; };
struct T2 { int a; };

template <typename T>
std::ostream & operator<<(std::ostream & os, const T & t2) 
{
    static_assert(!std::is_same<T2, T>::value, "Fail in << for T2");
    return os;
}

int main()
{
    T1 t1;
    T2 t2;
    std::cout << t1; // Works
    std::cout << t2; // Static assert fail
}

I'm not understanding. Doesn't this answer your question?

Upvotes: 0

rustyx
rustyx

Reputation: 85361

VS2012 has incomplete support of C++11. Default template arguments for function templates, being a C++11 feature, is supported starting with VS2013.

Maybe you can try this, but it is also using some C++11 features:

template <typename T>
auto operator<<(std::ostream & os, T const& t2) ->
    typename std::enable_if<std::is_same<T, T2>::value, std::ostream&>::type  
{
    static_assert(false, "Fail in << for T2");
    return os;
}

If that also doesn't work, I would just degrade gracefully for archaic crap like VS2012 and be done with it. 

#if _MSC_VER < 1800
std::ostream& operator<<(std::ostream & os, T2 const& t2); // will fail at link time
#else
. . .
#endif

Upvotes: 1

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