C++: How to test compile errors to check constraints

Question

I wrote constraints on C++ type using templates. Now I want to test them in my test suite, in order to notice when the behavior breaks.

Think about Field class like this.

template <size_t Size>
class Field {
  template<size_t Offset, size_t Count>
  Field<Count> SubString(typename std::enable_if<(Size >= Offset + Count)>::type* = 0) const {
    return Field<Count>(data_.substr(Offset, Count));
  }
}

Field<5>("abcde").SubString<3,4>() does not compile, because enable_if condition fails. I am looking for a way to check that.

I came up with writing a script to actually compile that expression and check return value, but I prefer to complete in C++. The compiler is g++ (GCC) 4.8.3 20140911. Compiler dependent methods are welcome.

Answer 1

"prefer to complete in C++"

I'm not aware of any general-purpose way to do this with any compiler. For your very specific example you could hack something up - e.g. a SubString overload that would match when enable_if fails, and static assert on the returned type's size.

But for more complex testing, it's normal to have a test "system" that compiles and runs various C++ programs with a variety of data, sometimes ensuring they terminate normally, other times that they can't be compiled etc.. IMHO, you might as well start working on this rather than convoluted hackery for a specific test, as it will prove more useful and flexible longer term.