Question

stringstream always seems to fail when I call stringstream::ignore(), even if this is done after calling stringstream::clear():

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <cassert>

using namespace std;

int main() {
    int a, b;
    stringstream ss;
    string str;
    ifstream inFile("file.txt");
    if(!inFile) {
        cerr << "Fatal: Cannot open input file." << endl;
        exit(1);
    }

    while(getline(inFile, str)) {
        ss << str;                // read string into ss
        ss >> a >> b;             // stream fails trying to store string into int

        ss.clear();               // reset stream state
        assert(ss.good());        // assertion succeeds

        ss.ignore(INT_MAX, '\n'); // ignore content to next newline
        assert(ss.good());        // assertion fails, why?
    }

    return 0;
}

file.txt contains the following text:

123 abc
456 def

Why is ss.good() false after ss.ignore()?

Answer 1

std::endl outputs \n and flushes the stream. However, stringstream::flush() is meaningless and does nothing. flush only has meaning when the underlying buffer is tied to an output device like the terminal, however, a stringstream has nowhere to flush the contents to. If you want to clear the contents of a stringstream do ss.str(""); instead. However, I would probably change the code to the following:

while(getline(inFile, str)) {
    ss.str(str);              // call ss.str() to assign a new string to the stringstream
    if(!ss >> a >> b)         // check if stream fails trying to store string into int
    {
        ss.clear();           // Read failed, so reset stream state
    }
    else
    {
        // Read successful
    }
    // Do other stuff
}

Also, if you want to insert a newline into the stringstream, just do ss << '\n'; and do not call std::endl.