Thread: stream seekp doesn't work with trailing space

Could the eofbit be set because it hit the end of the string?
Test ss.eof() to see.
Call ss.clear() to reset so the seekp will (hopefully) work.

The eofbit is set when the end of the stream (string) is encountered. When there's extra space after the number, the number is read without the eof being detected. When there's no space the eof is detected and the eofbit is set so that the next read will fail. With normal textual stream input you have at least a newline between the data and the actual eof.

In the example below, stringstream and fstream act the same way.

Code:

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

void g(iostream& ios, int off) {
    int n;
    ios.seekg(off);
    ios >> n;
    cout << n << '\n';
    if (ios.eof()) {
        ios.clear();
        cout << "eof\n";
    }
    ios.seekp(off);
    ios << n * 2;
}

void f(iostream& ios) {
    g(ios, 3);
    g(ios, 9);
}

int main() {
    string s = "abc123def456";

    // create file with string s and no newline
    fstream fs("f123.txt", ios_base::out);
    fs << s;
    fs.close();

    // try stringstream
    stringstream ss(s);
    cout << ss.str() << '\n';
    f(ss);
    cout << ss.str() << '\n';

    // try fstream
    fs.open("f123.txt", ios_base::in|ios_base::out);
    f(fs);
    fs.close();

    // display file
    fs.open("f123.txt");
    fs >> s;
    cout << s << '\n';
}