why std::scan_is emits runtime error in visual studio compiler? - c++

Question

The example here emits a runtime error with memory access violation in Visual Studio 2013.
#include <locale>
#include <iostream>
#include <iterator>
int main()
{
auto& f = std::use_facet<std::ctype<char>>(std::locale(""));
// skip until the first letter
char s1[] = " \t\t\n Test";
const char* p1 = f.scan_is(std::ctype_base::alpha, std::begin(s1), std::end(s1));
std::cout << "'" << p1 << "'\n";
// skip until the first letter
char s2[] = "123456789abcd";
const char* p2 = f.scan_is(std::ctype_base::alpha, std::begin(s2), std::end(s2));
std::cout << "'" << p2 << "'\n";
}
Why is that? Wrong implementation from the compiler?

Answer 1

The line auto& f = std::use_facet<std::ctype<char>>(std::locale("")); causes the error. The reference f is an alias for a null object. It seems that this implementation works for gcc C+11 and above compilers but it does not work in Microsoft compilers. Thus the right implementation in Visual Studio 2013 & 2015 which i tested, is:
#include "stdafx.h"
#include <locale>
#include <iostream>
#include <iterator>
int main()
{
std::locale loc(std::locale(""));
//auto& f = std::use_facet<std::ctype<char>>(std::locale(""));
auto& f = std::use_facet<std::ctype<char>>(loc);
// skip until the first letter
char s1[] = " \t\t\n Test";
const char* p1 = f.scan_is(std::ctype_base::alpha, std::begin(s1), std::end(s1));
std::cout << "'" << p1 << "'\n";
// skip until the first letter
char s2[] = "123456789abcd";
const char* p2 = f.scan_is(std::ctype_base::alpha, std::begin(s2), std::end(s2));
std::cout << "'" << p2 << "'\n";
}