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How come istringstream can't seem to fully read numeric literals with suffixes?

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

int main() {
    long long x = 123ULL; // shows 123ULL is a valid long long literal

    istringstream iss("123ULL");

    iss >> x;
    cout << "x is " << x << endl;

    char extra;
    iss >> extra;
    cout << "remaining characters: ";
    while(!iss.eof())
    {
        cout << extra;
        iss >> extra;
    }
    cout << endl;

    return 0;
}

The output of this code is 

x is 123
remaining characters: ULL

Is this behavior controlled by the locale? Could anyone point me to clear documentation on what strings are accepted by istringstream::operator>>(long long)?

Cœur
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ricab
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1 Answers1

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Yes, it's controlled by the locale (via the num_get facet), but no locale I ever heard of supports C++ language literals, and it would be the wrong place to customize this.

Streams are for general-purpose I/O, and C++ integer literal suffixes are very specialized.

The exact behavior of the default num_get facet is described in the C++11 standard in section 22.4.2.1. The description partially references the strto* family of functions from the C standard library. You can find a somewhat condensed version here:

http://en.cppreference.com/w/cpp/locale/num_get/get

Sebastian Redl
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  • Thanks. Could you advise on the standard way to interpret C++ numeric literals in C++? – ricab Dec 10 '13 at 16:13
  • You would write a wrapper type (`struct CppInt` or so) and overload `operator >>` for it. You might want to add information about the suffix to the struct, e.g. it could have a `bool unsigned` and `enum Type type` (for the suffixes `L` and `LL`), if your program cares about that. But if you're parsing something that has suffixed integer literals, streams are probably the wrong way to parse this at all; you should look into parser generators like Boost.Spirit. – Sebastian Redl Dec 10 '13 at 16:53