Assign a nullptr to a std::string is safe?

Question

I was working on a little project and came to a situation where the following happened:

std::string myString; #GetValue() returns a char* myString = myObject.GetValue(); 

My question is if GetValue() returns NULL myString becomes an empty string? Is it undefined? or it will segfault?

Answer 1

Interesting little question. According to the C++11 standard, sect. 21.4.2.9,

basic_string(const charT* s, const Allocator& a = Allocator()); 

Requires: s shall not be a null pointer.

Since the standard does not ask the library to throw an exception when this particular requirement is not met, it would appear that passing a null pointer provoked undefined behavior.

Answer 2

It is runtime error.

You should do this:

myString = ValueOrEmpty(myObject.GetValue()); 

where ValueOrEmpty is defined as:

std::string ValueOrEmpty(const char* s) {     return s == nullptr ? std::string() : s; } 

Or you could return const char* (it makes better sense):

const char* ValueOrEmpty(const char* s) {     return s == nullptr ? "" : s;  } 

If you return const char*, then at the call-site, it will convert into std::string.