C++ ifstream pointer open files fails

Question

It seems like ifstream*->open doesn't work as I expected... Here is the code: ( compiled with g++ 4.7 using -std=c++11 in MAC OSX 10.7)

#include <string>
#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
int main(int argc, char** argv)
{
    string line;
    vector<string> fname = {"a.txt","b.txt"};
    vector<ifstream*> files ( 2, new ifstream );

    files[0]->open( fname[0] );
    getline( *files[0], line,  '\n');
    cerr<<"a.txt: "<<line<<endl; 
    //this one prints the first line of a.txt

    line.clear();

    files[1]->open( fname[1] );
    getline( *files[1], line, '\n'); 

    cerr<<"b.txt: "<<line<<endl;
    //but this one fails to print any from b.txt
    //actually, b.txt is not opened!

    return 0;
}

Can any one tell me what is wrong here???

You realize that none of those files are being closed properly because you are storing pointers to dynamically allocated streams and not calling delete on them, right? — Ed Swangren
– Ed Swangren, Commented Sep 17, 2012 at 17:28
Do you mean ifstream->open instead of ifstream*->open which implies two de-references of the pointer (implied) by ifstream? Asked instead of edited to get answer. — Jonathan Seng
– Jonathan Seng, Commented Sep 17, 2012 at 19:00

Jonathan Seng · Accepted Answer · 2012-09-17 17:35:36Z

5

This executes new std::ifstream once where used, not once per 2 values you requested.

The new std::ifstream creates an ifstream pointer whose pointer value is inserted two times in files by the std::ifstream constructor.

std::vector takes care of only the objects it contains, which in this case are ifstream* pointers. So, the 2 copies the pointer value. When files falls out of scope, the pointers (and supporting data in the vector) are taken care of, but not the values the pointers point to. As such, vector does not delete your new std::ifstream object (placed in the vector twice).

operator delete is not called for you because pointers can have many uses which cannot be determined easily. One of which is to put the same pointer in the vector twice on purpose.

edited Sep 17, 2012 at 17:35

answered Sep 17, 2012 at 17:30

Jonathan Seng

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