can't call ifstream in a separate function

Question

I am learning filestreams. I have a problem while using ifstream in a separate function outside main. Inside main it works just fine. the function is such,

void counter(string str)
 {
    ifstream inpufile(str,ios::in);//the error is here, it says the function cannot be called!?!
    if(!inpufile)
    {
        cout<<"\n The file does not exist, try again";
    }
    else
    {
        char c;
        int counter=0;
        while(inpufile.get(c))
        {
            if(c=='\n')
                counter++;
        }
        cout<<"\n The number of lines in the file are "<<counter+1;
    }
    inpufile.close();
 }

The function call is from main() and is such

counter(argv[1]);

Can I only pass ifstream as an object. Can I not create one outside main?

Answer 1

Your problem doesn't have anything to do with the function, it has to do with the variable holding the filename. Consider:

int main(int argc, const char** argv)
{
    std::ifstream inpufile(argv[1], ios::in); // ok

    std::string fname = argv[1]; // ok
    std::ifstream fileb(str, ios::in); // fails pre-C++11
}

By having a function, you're causing an implicit conversion from const char* to std::string, just like in the example above. And std::fstream didn't take std::string for the filename until C++11. Two ways to fix:

void counter(const char* fname)
{
    ifstream inpufile(fname, ios::in); // works now

and counter(argv[1]); still works, in fact it works a tiny bit better because no conversion is needed.

Or

void counter(std::string fname)
{
    ifstream inpufile(str.c_str(), ios::in); // ok too

Which gets the const char* that fstream expects.

C++11 does finally fix this and let you use std::string directly.