Question about using; - Page 2

HelloWorld · #11 (**permalink**) 07-26-2007, 03:39 PM

Quote:

you can use global variables to affect your entire file

So for example if I declare:

Code:

using std::cin;

in test.cpp
I don't have to declare it again in any of my other files within that solution or folder (where test.cpp is at..)? Or how does it exactly works?

Bench · #12 (**permalink**) 07-26-2007, 04:02 PM

Quote:

Originally Posted by HelloWorld

So for example if I declare:

Code:

using std::cin;

in test.cpp
I don't have to declare it again in any of my other files within that solution or folder (where test.cpp is at..)? Or how does it exactly works?

No, only in that one file. You could potentially get name clashes in test.cpp if you created a variable or function called 'cin', because the name would already be reserved. It wouldn't necessarily have any effect on any other files. (Although it can, if you're doing it in a .h header file) The main reason for namespaces is to stop name clashes from happening. One thing that 'using' does is un-do the name hiding which namespaces provide.

Essentially, its a tool for laziness. since you can still access 'cin' regardless, by prefixing the name with std::

here's a program with a 'using' directive (the lazy way)

Code:

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string my_str = "Hello, World";
    cout << my_str << endl;
}

And here's one without (the un-lazy way)

Code:

#include <iostream>
#include <string>

int main()
{
    std::string my_str = "Hello, World";
    std::cout << my_str << std::endl;
}

rpgfan3233 · #13 (**permalink**) 07-26-2007, 04:42 PM

using is similar to import in that you need to declare it in every file you want to use it in. For example, in Java you can use import java.io.*; in a MsgSend class. Then in a MsgRecv class, you would need to use import java.io.*; again, right? The same is true in C++. If you have a file test.cpp, using std::cin; is only valid for test.cpp. In other words, another file such as main.cpp would have no knowledge of that using declaration. It is all about the scope of various items. If you used using std::cin; inside main(), you would only be able to use cin in main(). If you tried to use cin in another function, you would receive an compile error. You would either need to use using std::cin; before actually using cin inside the function or you would need to use the fully-qualified name std::cin for every time that you used cin. Example:

Code:

#include <iostream>

// Answer To Life, the Universe and Everything test
bool answerCorrect (int answer) {
    if (answer != 42)
        return false;
    return true;
}

// asks user to decide to try again or not
int tryAgain () {
    int answer = 'q';
    std::cout << "If you wish to quit, type 'q' and press Enter." << std::endl;
    std::cout << "Otherwise, type another character and press Enter. ";
    answer = std::cin.get();
    if (!std::cin.good())
        std::cin.clear();
    std::cin.sync();
    return answer;
}

int main () {
    using std::cin;
    using std::cout;
    using std::cerr;
    using std::endl;

    int ans = 0;
    do {
        cout << "What is the answer to Life, the Universe and Everything? ";
        cin >> ans;
        if (!cin.good()) { //invalid input such as 'a' instead of a number
            cin.clear();
            cerr << "Your answer was invalid." << endl << endl;
        }
        else if (!answerCorrect(ans))
            cout << "Sorry, but your answer was incorrect." << endl << endl;
        else {
            cout << "Correct!" << endl;
            break; //exit the loop so the program can end
        }
        cin.sync(); //discard leftover characters in the input buffer
    } while (tryAgain() != 'q');
    return 0;
}

Simple code, but effective. I apologize for the formatting when the input is invalid or someone wants to play again. However, it was just something quick to illustrate the purpose.

Wherever you put using, whether it is globally (outside of everything) or within a function such as main() or whatever, it is confined to that file and further confined to the block of code it is enclosed by, if it is not global.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

The Following User Says Thank You to Bench For This Useful Post:
HelloWorld (07-26-2007)

The Following User Says Thank You to rpgfan3233 For This Useful Post:
HelloWorld (07-26-2007)