SFINAE + sizeof = detect if expression compiles

Question

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

template T& lvalue_of_type();
template

        

Answer 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 :-)

Answer 2

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.