Question

Is OK to throw an exception when 0 is passed to allocate method?

Thank you.

P.S.

If n == 0, the return value is
unspecified.

Does it mean that allocate shouldn’t throw an exception? I am inclined to think that if throwing wasn’t allowed for n==0, then the standard would clearly spell it out.

Answer 1

All the standard has to say (§20.1.5/2) is that given

• T – any type
• X – an Allocator class for type T
• a – a value of type X&
• n – a value of type X::size_type,

the return value of the expression a.allocate(n) is unspecified if n equals 0.

One one hand, given that X::allocate doesn’t have any mandated exception specification and explicitly may throw std::bad_alloc, I don’t see why it couldn’t throw some other type of exception as well. On the other hand, the wording specifically calls out the condition wherein n equals zero and directly implies that there is a return value, meaning you shouldn’t throw. I think it may be open to some interpretation, but personally I’d side with the latter and consider it an unexceptional code-path.