views:

180

answers:

3

Here's probably a very noobish question for you: How (if at all possible) can I return an ifstream from a function?

Basically, I need to obtain the filename of a database from the user, and if the database with that filename does not exist, then I need to create that file for the user. I know how to do that, but only by asking the user to restart the program after creating the file. I wanted to avoid that inconvenience for the user if possible, but the function below does not compile in gcc:

ifstream getFile() {
    string fileName;
    cout << "Please enter in the name of the file you'd like to open: ";
    cin >> fileName;
    ifstream first(fileName.c_str());
    if(first.fail()) {
        cout << "File " << fileName << " not found.\n";
        first.close();
        ofstream second(fileName.c_str());
        cout << "File created.\n";
        second.close();
        ifstream third(fileName.c_str());
        return third; //compiler error here
    }
    else
        return first;
}

EDIT: sorry, forgot to tell you where and what the compiler error was:

main.cpp:45: note: synthesized method ‘std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream(const std::basic_ifstream<char, std::char_traits<char> >&)’ first required here 

EDIT: I changed the function to return a pointer instead as Remus suggested, and changed the line in main() to "ifstream database = *getFile()"; now I get this error again, but this time in the line in main():

main.cpp:27: note: synthesized method ‘std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream(const std::basic_ifstream<char, std::char_traits<char> >&)’ first required here
+3  A: 

ifstream does not support copy construct semantics (that what the error message basically sais), so you cannot return an ifstream. Return an ifstream* instead, and pass to the caller the responsability to delete the allocate pointer.

Remus Rusanu
wrongusername
Remus Rusanu
You mean, something like "ifstream returnStream = return returnStream;"?btw, that doesn't work either. it gives me an error: "main.cpp:56: error: conversion from ‘std::ifstream*’ to non-scalar type ‘std::ifstream’ requestedmain.cpp:57: error: invalid conversion from ‘void*’ to ‘std::ifstream*’"
wrongusername
Wrong, I very seriously and strongly suggest you read up on C/C++ memory management before you continue. There are just way too many things you don't know.
Remus Rusanu
Thanks! sure...I had a long time ago, but must've forgotten most of it :)
wrongusername
+4  A: 

No, not really. ifstream doesn't have a copy constructor, and if you try to return one, that means copying the instance in your function out to wherever the return needs to go.

The usual workaround is to pass in a reference to one, and modify that reference in your function.

Edit: while that will allow your code to work, it won't fix the basic problem. Right now, you're mixing two rather different responsibilities into a single function: 1) obtain a file name, 2) open or create that file. I think if you separate those, the code will be simpler, and make it much easier to eliminate the source of the problem you're seeing.

Edit 2: Using a reference like this works perfectly well without an operator=. The general idea is something like:

int open_file(char const *name, fstream &stream) { 
    stream.open(name);
}

The assignment operator is neither necessary nor useful in this case -- we simply use the existing fstream via the reference. An operator= would be necessary if and only if we had to pass the argument to the ctor. With a stream, we can default construct a stream that doesn't connect to a file, and then use open to connect to the file after the fact.

Jerry Coffin
wrongusername
but I think it did get me down to that one error, though.
wrongusername
The "trick" of passing in a reference to avoid returning a new object will only work for types that have an `operator=`. std::ifstream doesn't have that either, so this advice won't work for you.
MSalters
+3  A: 
Corwin
Thanks!!! after a bit of tinkering with your code I finally got what I wanted. :D
wrongusername