Re: std::ifstream

From:

=?Utf-8?B?Rmls?= <Fil@discussions.microsoft.com>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 19 Jun 2008 12:35:00 -0700

Message-ID:

<9D37ED56-0EEB-4544-8ABC-3A448B1E83C0@microsoft.com>

"David Wilkinson" wrote:

Fil wrote:

Hi,

I am learing C++ with Thinking in C++ (Bruce Eckel) and we've done some
exercises on ifstream in the past chapter.
If I pass to my ifstream object a constant filePath (as we did in he book)
it works, but it doesn't as soon as I pass it a variable.
I have got one error araising from linesCount.cpp, line 6.

main.cpp
-------------------------------------------------------------------------------------
#include "linesCount.h"

int main()
{
    return linesCount("C:\\TICPP-2nd-ed-Vol-one\\html\\Frames.html");
}
-------------------------------------------------------------------------------------

linesCount.h
-------------------------------------------------------------------------------------
#include <string>
int linesCount(std::string filePath);
-------------------------------------------------------------------------------------

linesCount.cpp
-------------------------------------------------------------------------------------
#include <string>
#include <fstream>

int linesCount(std::string filePath)
{
    std::ifstream inFile(filePath);
    std::string line;
    int count(0);
    while (std::getline(inFile,line))
    {
        count++;
    }
    return count;
}
-------------------------------------------------------------------------------------

The error message is:

linescount.cpp(6) : error C2664:
'std::basic_ifstream<_Elem,_Traits>::basic_ifstream(const char
*,std::ios_base::openmode,int)' : cannot convert parameter 1 from
'std::string' to 'const char *'
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        No user-defined-conversion operator available that can perform this
conversion, or the operator cannot be called

Fil:

You have to use c_str() member to get const char* from a string;

Thanks a lot. I trust that ifstream will be able to find the length of the
string by itself.
Usually when I am passing an array to a function through a pointer I also
pass the length.

int linesCount(const std::string& filePath)
{
    std::ifstream inFile(filePath.c_str());
    std::string line;
    int count(0);
    while (std::getline(inFile,line))
    {
        count++;
    }
    return count;
}

That's cool, it works perfectly.

Note that you should pass the string by const reference here, to avoid
unnecessary copy.

I am not sure why but I hope it is explained somewhere in my textbook
("Thinking in C++"). Thanks again.

--
David Wilkinson
Visual C++ MVP