SFINAE + sizeof = detect if expression compiles

Question

I just found out how to check if operator<< is provided for a type.

template<class T> T& lvalue_of_type();
template<class T> T  rvalue_of_type();

template<class T>
struct is_printable
{
    template<class U> static char test(char(*)[sizeof(
        lvalue_of_type<std::ostream>() << rvalue_of_type<U>()
    )]);
    template<class U> static long test(...);

    enum { value = 1 == sizeof test<T>(0) };
    typedef boost::integral_constant<bool, value> type;
};

Is this trick well-known, or have I just won the metaprogramming Nobel prize? ;)

EDIT: I made the code simpler to understand and easier to adapt with two global function template declarations lvalue_of_type and rvalue_of_type.

With VC++ it appears that `is_printable::value` is true for any X, and with Comeau Online it appears to be false for any X. — UncleBens, Jan 24 '10 at 21:32
With g++, I get 1 for `is_printable::value` and 0 for `is_printable >::value`, so it works fine for me. — fredoverflow, Jan 24 '10 at 22:30
That's shorter: ```template static auto check(X& x, Y& y) -> decltype(x << y);``` — vvassilev, Jul 19 '17 at 21:40

score 7 · Accepted Answer · edited May 23 '17 at 12:34

It's a well known technique, I'm afraid :-)

The use of a function call in the sizeof operator instructs the compiler to perform argument deduction and function matching, at compile-time, of course. Also, with a template function, the compiler also instantiates a concrete function from a template. However, this expression does does not cause a function call to be generated. It's well described in SFINAE Sono Buoni PDF.

Check other C++ SFINAE examples.

Alexander Gessler · Answer 2 · 2010-01-24T16:33:07.757

1

It is just a combination of two well-known tricks. SFINAE says 'substitution failure is not an error' - that's exactly what you did. Using sizeof to let the compiler substitute template arguments into an expression without actually executing it is also common.

Sorry :-)

edited Jan 24 '10 at 16:33

answered Jan 24 '10 at 16:14

Alexander Gessler

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SFINAE + sizeof = detect if expression compiles

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