Question

From Stroustrup’s TC++PL, 3rd Edition, Section 21.3.3:

If we try to read into a variable v and the operation fails, the value of v should be unchanged (it is unchanged if v is one of the types handled by istream or ostream member functions).

The following example appears to contradict the above quote. Based on the above quote, I was expecting the value of v to remain unchanged — but it gets zeroed. What’s the explanation for this apparent contradictory behaviour?

#include <iostream>
#include <sstream>

int main( )
{
    std::stringstream  ss;

    ss  << "The quick brown fox.";

    int  v = 123;

    std::cout << "Before: " << v << "\n";

    if( ss >> v )
    {
        std::cout << "Strange -- was successful at reading a word into an int!\n";
    }

    std::cout << "After: " << v << "\n";

    if( ss.rdstate() & std::stringstream::eofbit  ) std::cout << "state: eofbit\n";
    if( ss.rdstate() & std::stringstream::failbit ) std::cout << "state: failbit\n";
    if( ss.rdstate() & std::stringstream::badbit  ) std::cout << "state: badbit\n";

    return 1;
}

The output I get using x86_64-w64-mingw32-g++.exe (rubenvb-4.7.2-release) 4.7.2 is:

Before: 123
After: 0
state: failbit

Thanks.

Answer 1

From this reference:

If extraction fails (e.g. if a letter was entered where a digit is expected), value is left unmodified and failbit is set (until C++11)

If extraction fails, zero is written to value and failbit is set. If extraction results in the value too large or too small to fit in value, std::numeric_limits::max() or std::numeric_limits::min() is written and failbit flag is set. (since C++11)

It seems that your compiler is compiling in C++11 mode, which changes the behavior.

---

The input operator uses the locale facet std::num_get whose get function invokes do_get. For C++11 it’s specified to use std::strtoll et. al. type of functions. Before C++11 it apparently used std::scanf style parsing (going by the reference, I don’t have access to the C++03 specification) to extract the numbers. The change in behavior is due to this change in parsing the input.