Diagnosable validity

by Andrzej Krzemieński

From the article:

Certain combinations of types and expressions can make a C++ program ill-formed. “Ill-formed” is a term taken from the C++ Standard and it means that a program is not valid, and compiler must (in most of the cases) reject it. This is quite obvious:

int main()
{
  auto i = "some text".size(); // invalid expression
};

String literals do not have member functions, therefore compiler cannot accept this program, and must report an error. This puts a responsibility on programmers to learn which expressions and types are valid in a given context and use only these. Again, I am saying a very obvious thing.

What is less obvious is that there is a way in C++ to enter a type or expression of which we do not know if it is valid or not, in an isolated environment, where it does not render the entire program ill-formed, but instead it returns a yes-no (or rather valid-invalid) answer, which we can use at compile-time to make a decision how we want the program to behave. When requested, compiler can analyze all the declarations it has seen so far, and make an approximated judgement whether a given type or expression would make the program ill-formed or not, if used outside the isolated environment. The compiler’s approximated answer is not always correct, but it is just enough most of the time.