ifstream + opening random txt file (c_str)

Question

I want to open a random .txt file and put the data into some strings. It works if I write the path into the code.

I don't get it why this doesn't work.

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    string file;

    ifstream filein(file.c_str());

    cout << "Insert Path" << endl;
    cin >> file;

    cout << file << endl;

    filein.open(file.c_str(), ios::in);

    for (string line; getline(filein, line);) {
        cout << line << endl;
    }

    return 0;
}

When you ran over this in a debugger, what was `file`? What is it supposed to be? — Fantastic Mr Fox, Jan 06 '17 at 19:34

score 2 · Answer 1 · answered Jan 06 '17 at 19:37

2

Your filename string is empty because std::string defaults to empty.

You are passing an empty string (or the nul string) to the ifstream constructor, which is at best, undefined behavior.

answered Jan 06 '17 at 19:37

Thomas Matthews

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score 0 · Answer 2 · edited May 23 '17 at 10:30

Try writing your code like this:

#include <iostream>
#include <fstream>

int main()
{
    std::string file;

    std::cout << "Insert Path" << std::endl;
    std::getline(std::cin, file);

    std::cout << file << std::endl;

    std::ifstream filein(file);

    for (std::string line; std::getline(filein, line); )
    {
        std::cout << line << std::endl;
    }

    return 0;
}

Notable edits include:

We're now constructing the ifstream object only when we need it, after file has had data stored, which means no more undefined behavior, and that we only attempt to open a file after we know what the path is.
We're retrieving a whole line when storing to file, instead of only the first word, which is crucial if your path includes any spaces.
We're just using the file string directly. There's no need to call c_str().
We're no longer using using namespace std;. There are many, many reasons why this is bad practice.

EDIT:

If you have a C++17-compliant compiler, I'm going to propose you write code that looks like this instead:

#include <iostream>
#include <fstream>
//You may need to write #include <experimental/filesystem>
#include <filesystem>
#include <string>

int main()
{
    std::string input_line;

    std::cout << "Insert Path" << std::endl;
    std::getline(std::cin, input_line);

    //You may need to write std::experimental::filesystem
    std::filesystem::path file_path{input_line};
    //This will print the "absolute path", which is more valuable for debugging purposes
    std::cout << std::filesystem::absolute(file_path) << std::endl;

    std::ifstream filein(file_path);

    for (std::string line; std::getline(filein, line); )
    {
        cout << line << endl;
    }

    return 0;
}

Explicit use of path objects will make your code more readable and make errors more explicit, as well as grant you access to behavior you otherwise would not be able to access.

So, why should anybody write code like that? What does this do for the OP? Please edit your answer with clarifications, such as the difference between your answer code and the OP's. — Thomas Matthews, Jan 06 '17 at 19:39
Why even default construct the `ifstream`? You can easily make it work using `std::string filename; std::cin >> filename; std::ifstream fin(filename)`. — NathanOliver, Jan 06 '17 at 19:42
Remember variables should be declared when you need them, which is where they are used, not at the top of the function. — NathanOliver, Jan 06 '17 at 19:43
So ok i understand your suggestions. The declaration in the second row makes no sense. The path has no spaces. So it should work either way. But the code Xirema postet doesn't work for me. I'm using CodeBlocks 16 with gnu compiler. — TF107, Jan 06 '17 at 19:53
@TF107 When you say "It doesn't work", what precisely do you mean? Are you getting a compilation error? Are you getting a runtime error? Is the output incorrect? — Xirema, Jan 10 '17 at 19:52

score 0 · Answer 3 · answered Jan 06 '17 at 22:09

first what are you opening? as long as you string doesn't contain anything??

second even if the string contains a valid path and the opening was successfull at the first time but in the second will fail as long as you use the same file stream on multiple files without clearing its buffer and closing the previous file:

string file "C:\\MyProject\\data.txt"; // let's say a valid path

ifstream filein(file.c_str());

if(filein.fail()) // the condition fails as long as the opening was successfull
    cout << "failed to open file!" << endl;

cout << "Insert Path" << endl;
cin >> file; // let's say the user enters a valid path again: "C:\\MyProject\\test.txt"

cout << file << endl;

filein.open(file.c_str(), ios::in); // fail to correct it:

filein.close();
filein.clear(); // very important

filein.open(file.c_str(), ios::in); // now it's ok!

for (string line; getline(filein, line);) {
    cout << line << endl;
}

ifstream + opening random txt file (c_str)

3 Answers3

EDIT: