Why isn't it a compile-time error to return a nullptr as a std::string?

Question

Due to a bug, I just found out that this code compiles fine on with Visual Studio 17 and probably on other compilers as well. Now I'm curious why?

#include <iostream> #include <string> std::string foo(){ return nullptr; } int main(){ auto s = foo(); std::cout << s << std::endl; }

I could imagine it is because the std::basic_string c'tor could be invoked with a char* and while returning an implicit conversion from ptr to std::string occurs (with NULL as argument and then goes poof). Am I on the right way?

it is probably indeed passing nullptr as the parameter for a pointer — Tarick Welling
– Tarick Welling, Commented Jun 17, 2019 at 11:42
It would probably be beneficial to add a string (nullptr_t) = delete ctor, just so this error is caught at compile time. I don't think legitimate programs would break due to that. — MSalters
– MSalters, Commented Jun 17, 2019 at 12:03
Related, Why does constructing std::string(0) not emit a compiler warning?, Access Violation when sending a 0 int literal to a const string parameter, Avoiding improper std::string initialization with NULL const char* using g++, How to best protect from 0 passed to std::string parameters?, etc. — jww
– jww, Commented Jun 18, 2019 at 0:37

Hatted Rooster · Accepted Answer · 2019-06-17 11:53:03Z

Yes, your assumption is right, checking std::basic_string constructors #5 will be called:

basic_string( const CharT* s, const Allocator& alloc = Allocator() );

Note that passing nullptr invokes undefined behavior as stated in the standard and the notes :

The behavior is undefined if [s, s + Traits::length(s)) is not a valid range (for example, if s is a null pointer).

Interesting enough, std::string_view's constructor has attribute__((__nonnull)) and the compiler emits a warning when NULL or nullptr is passed to the std::string_view's constructor. I wonder, why there is no such attribute for std::string's constructor

Evg · Accepted Answer · 2019-06-17 12:16:49Z

Why shouldn't it compile? std::string has the following constructor:

string(const CharT* s, const Allocator& alloc = Allocator());

that constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s. The constructor is not explicit, so the implicit conversion from nullptr to std::string is indeed possible.

@RichardCritten, it definitely is. But the question was why this code compiles, not what happens if the code is executed. UB will happen not only for nullptr, but for any pointer that doesn't point to the beginning of a null-terminated string.

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