Converting a steam from an input file to a String

byteframe · 10-10-2005, 05:12 PM

I have some bigass input stream from a file....

string filename;
cout << "Enter the file name to decrypt : ";
cin >> filename;
ifstream input(filename.c_str());

Ill be needing many of the various functions from the string library to help me work through the text, so I'd like to take the stream input and put it into a new string. I can then work more comfortably from there. Is their anyway to do this? I've been having trouble with alot of other ideas, and my knowledge of streams is pretty weak.

But please help if you can, thank you very much.

taylor_venable · 10-10-2005, 06:18 PM

I would use an output string stream. That way you can stream data character-by-character from the input file, and into the ostrstream. When you're done, you can "freeze" the ostrstream into a standard string. Here's an example that reads everything from a file, then puts it into an ostrstream, and prints it out as a string:

Code:

#include <fstream>
#include <iostream>
#include <strstream>

int main(int argc, char** argv) {
    std::string filename;
    std::ostrstream* oss = new std::ostrstream();
    std::cout << "FILENAME:";
    std::cin >> filename;
    std::ifstream input(filename.c_str());
    char c;
    while(input.get(c)) {
        *oss << c;
    }
    std::cout << "FILE CONTENTS: " << oss->str();
    return 0;
}

The str() method returns the standard string form of what's in the ostrstream. Calling str() on an ostrstream freezes it. If you want to start putting more data back into oss after you call the str() method on it, you have "thaw" it first, by calling this method:

Code:

oss->freeze(false);

byteframe · 10-10-2005, 06:46 PM

So how do I make a new string and put the stuff in it?

taylor_venable · 10-10-2005, 07:10 PM

You don't have to read data into a string; instead, you can read it into a stream, and then convert that into a string when you're finished getting the data from the file. An ostrstream is a special type of stream that you can write stuff into (just like you would cout) and when you're done, get a string back. Just use the put-to operator (<<) to put stuff into the stream. When you're finished, calling str() on it will return a string. Thus, to get a string object for use later on, use:

Code:

std::string* s = new std::string(oss->str());

If you want to continue to put into an ostrstream after you use str(), you have to call freeze(false) on it first, but everything you had put into it up until that point will still be there.

byteframe · 10-10-2005, 07:32 PM

I see. What is * and why do I need it in my string declaration, and whats ->?

taylor_venable · 10-10-2005, 07:57 PM

The result of using the "new" operator is a pointer, or (in Java-speak) a reference. It's a memory address where the real data can be found. The star (*) means, "follow the pointer". In other words, using "*oss" means, "go directly to the data represented by oss". It's the only way to get at what was created when you use the "new" operator.

The arrow operator does something similar. Normally you access an object with the dot, like "foo.print()"; but if foo is a pointer, you'd have to do "(*foo).print()". [You need the star inside parenthesis because of operator precedence.] The arrow is a shortcut: you use it instead of a dot, and the object is automatically dereferenced. So, "(*foo).print()" is the same as "foo->print()".

I was just looking into the use of ostrstream a little more in one of my own projects, and realized that it's actually not supposed to be used anymore. Instead, the preferred method uses a "ostringstream" object instead (found in the "sstream" header). You can just substitute that into my code above, in lieu of "ostrstream".

I guess since the default constructor has no parameters, you could just avoid using "new" altogether. Then you wouldn't have access to a pointer, but directly to the object itself. Then you can use just a plain dot, and no star. Expanding my above code, you would get:

Code:

#include <fstream>
#include <iostream>
#include <sstream>
#include <string>

int main(int argc, char** argv) {
    std::string filename;
    std::ostringstream oss;
    std::string s;
    std::cout << "FILENAME:";
    std::cin >> filename;
    std::ifstream input(filename.c_str());
    char c;
    while(input.get(c)) {
        oss << c;
    }
    s = oss.str();
    std::cout << "FILE CONTENTS: " << s;
    return 0;
}

Pretty much any book about C or C++ should have more information about pointers, if you're interested. They can be complicated, but they're absolutely vital to do even a little bit of advanced C programming.

byteframe · 10-10-2005, 08:08 PM

Thank you I should be able to use this information.