Static Assert that a type A can be constructed from type B

Question

I am trying to to do a static assertion to check that for two types A and B; class A should have a public constructor of signature A(B& b) or A(const B& b).

I would like to have something of sort of is_constructable_from<A, B>::value which evaluates to true if there exists a public A(B& b) or A(const B& b) or A(B b).

I guess this is somewhat different than boost is_convertible type_trait.

How do I achieve this?

Have I missed something which will do this in Boost Type Traits or Concept Check library?

Right now I need this to do a static check that the expression A a(b) is a valid one, which as suggested by Sehe is a less restrictive than the previous concept. A solution for both the cases are welcome. I am allowed to use Boost.

Your "in other words" description describes a /different/ requirement! (implicit conversions and bound temporaries...) — sehe
Pardon my newbie c++ knowledge, but I didn't understand the temporary part of your comment. For now I will be happy with the 2nd description than the more strict 1st description. — iNFINITEi
If your ctor takes a const& or a value it can accept the result of implicit conversions (temporaries). — sehe
Okay, I updated my question. I would like to know the solution for both this concept checks. — iNFINITEi
Off the top of my head, if you can convert from B to A, then there is either A(B), A(B &), A(const B &), A(B&&), A(const B &&), or conversion operator(s) like: B::operator A(). So it's not clear to me what you want to disallow. — kec

mockinterface mockinterface · Accepted Answer · 2014-04-30T13:39:25

Constructors are not member functions - they don’t have an address that you can take, so it is impossible to test for their presence in a straightforward way with SFINAE^*.

While it isn't exactly what you asked for, with C++11 you can get close with std::is_constructible, and with C++03 you have to rollout your own:

template<class A, class B = void>
struct is_constructible;

template<class A, class B>
struct is_constructible { 

    template<class U> static U declval();    
    template<std::size_t N> struct dummy;
    template<class U> static char test(dummy<sizeof(U(declval<B>()))>*);        
    template<class U> static long test(...);

    static const bool value = sizeof(test<A>(0)) == sizeof(char); 
};

_{^* The following is an interesting attempt, but to the best of my understanding it doesn't work well across compilers:
Check at compile time class constructor signature}

Static Assert that a type A can be constructed from type B

1 Answers