Is it valid to pass nullptr to std::string::assign?

Question

I have a function which returns a pointer and a length, and I want to call std::string::assign(pointer, length). Do I have to make a special case (calling clear) when length is zero and the pointer may be nullptr?

The C++ standard says:

21.4.6.3 basic_string::assign  basic_string& assign(const charT* s, size_type n); Requires: s points to an array of at least n elements of charT. 

So what if n is zero? What is an array of zero characters and how does one point to it? Is it valid to call

s.assign(nullptr, 0); 

or is it undefined behavior?

The implementation of libstdc++ appears not to dereference the pointer s when the size n is zero, but that's hardly a guarantee.

Answer 1

Pedantically, a nullptr does not meet the requirements of pointing to an array of size >=0, and therefore the standard does not guarantee the behaviour (it's UB).

On the other hand, the implementation wouldn't be allowed to dereference the pointer if n is zero, because the pointer could be to an array of size zero, and dereferencing such a pointer would have undefined behaviour. Besides, there wouldn't be any need to do so, because nothing is copied.

The above reasoning does not mean that it is OK to ignore the UB. But, if there is no reason to disallow s.assign(nullptr, 0) then it could be preferable to change the wording of the standard to "If n is greater than zero, then s points to ...". I don't know of any good reason to disallow it, but neither can I promise that a good reason doesn't exist.

Note that adding a check is hardly complicated:

s.assign(ptr ? ptr : "", n); 

What is an array of zero characters

This is: new char[0]. Arrays of automatic or static storage may not have a zero size.