Not getting any warning when returning NULL as object

Question

I don't understand why I don't get a warning (with g++ or clang++) for returning a NULL as an object in newtstr() below:

#include<iostream>
using namespace std;

string newstr();

int main()
{
        string s = newstr();
        cout << s << endl;
        return 0;
}

string newstr()
{
        return NULL;
}

.

$ g++ -Wall -Wextra -pedantic testptr.cpp
$

I do, however, get a runtime error:

$ ./a.out
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid
Aborted
$

.

$ g++ --version
g++ (GCC) 4.8.0

Answer 1

std::string has a constructor basic_string( const CharT* s, const Allocator& alloc = Allocator() ); which accepts a const char* pointer for constructing a new string (see ⁽⁵⁾). The line return NULL; results in a implicit call to that constructor with a NULL pointer as argument - which is valid code but obviously won't run correctly.