what type-conversion is taking place when a boolean value used to construct a string object?

Question

In my code, there was a typo: instead of using "false" while initializing a std::string object, I typed false (which is a bool). Now this did not report any compilation error. But later in my code, when this string-object is being used, I get std::logic_error during runtime. Can anyone please explain, why the construction was allowed in this case (else I would have received a compilation error and found the problem there) ?

Here is a small snippet -

#include <iostream>

int main ()
{

   std::string str = false;

   std::cout << str << "\n";

}

The o/p that i get while running this -

xhdrdevl8@~/MYBACKUP=>g++ -o test_string -g test_string.cxx

xhdrdevl8@~/MYBACKUP=>./test_string

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct NULL not valid
Aborted

Answer 1

std::string has a constructor that takes a const char* to a null-terminated string.

false can be used as a null-pointer constant because it is an integral constant expression with a value of zero, so this std::string constructor is used.

Passing a null pointer to this constructor yields undefined behavior. Your Standard Library implementation helps you out here by generating a logic_error exception to inform you that you have violated the constraints of the std::string constructor by passing it a null pointer. Other implementations may not be so helpful (you might get an immediate crash or data corruption or who knows what).