Why explicitly delete the constructor instead of making it private?

Question

When/why would I want to explicitly delete my constructor? Assuming the reason is to prevent its usage, why not just make it private?

class Foo
         


        

Answer 1

I've met with default ctors declared as 'deleted' in the source code of LLVM (in AlignOf.h for instance). The associated class templates are usually in a special namespace called 'llvm::detail'. The whole purpose there I think was that they considered that class only as a helper class. They never intended to instantiate them; only to use them within the context of other class templates with some metaprogramming tricks that run in compile time.

Eg. there's this AlignmentCalcImpl class template which is used only within another class template called AlignOf as a parameter for the sizeof(.) operator. That expression can be evaluated in compile time; and there's no need to instantiate the template -> so why not declare the default ctor delete to express this intention.

But it's only my assumption.