Reading a double from an istream in Hex

Given double foo I can assign it from a hex format string using sscanf like this:

sscanf("0XD", "%lg", &foo)

But I cannot seem to get an istringstream to behave the same way. All of these just write 0 to foo:

1. istringstream("0XD") >> foo
2. istringstream("0XD") >> hex >> foo
3. istringstream("D") >> hex >> foo

This is particularly concerning when I read here that the double istream extraction operator should:

The check is made whether the char obtained from the previous steps is allowed in the input field that would be parsed by std::scanf given the conversion specifier

Why can't I read hex from an istream? I've written some test code here if it would be helpful in testing.

What you're looking for is the hexfloat modifier. The hex modifier is for integers.

On compliant compilers this will solve your problem.

#include <cstdio>
#include <iomanip>
#include <iostream>
#include <sstream>
using namespace std;

int main() {
    double foo;

    sscanf("0xd", "%lg", &foo);
    cout << foo << endl;

    istringstream("0xd") >> hexfloat >> foo;
    cout << foo << endl;

    istringstream("d") >> hexfloat >> foo;
   cout << foo << endl; 
}

Using Visual Studio 2015 will produce:

13
13
13

Using libc++ will produce:

13
13
0

So I'm not sure of the legitimacy of istringstream("d") >> hexfloat >> foo