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?

Barefoot

This question is pretty similar: #2408211 – Woodwork

Microsoft always had a bug where assigning a 0x0 to std::string was OK. By looking at your code it seems like you are a microsoft guy, so it might work for you. But.. otherwise it would trigger a SIGSEGV. – Ryder

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.

Bridgetbridgetown

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.

Duiker

Answer 3

Update:

Since C++23 adopted P2166, it is now forbidden to construct std::string from nullptr, that is, std::string s = nullptr or std::string s = 0 will no longer be well-formed.

Blackshear

This answer seems to refer to initialization at compile time, but it does not tell what will happen when the nullptr is returned from a function where the compiler cannot deduce the value at compile time (as scetched in the question), – Inexplicable

So now our constructors have to look like this trash "MyClass( const char *internal_name ) : name{ internal_name ? internal_name : "" } {} Yet another reason to avoid the everything everywhere all at once "standard" library. – Behistun

Answer 4

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

It's undefined behavior. The compiler and run time can do whatever it wants and still be compliant.

Waitress