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Posted 16 Nov 1999

Saving a variable temporarily

, 16 Nov 1999
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A safe, and convenient way to store variables temporarily
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    On several occassions I've had to execute a piece of code which used a global variable that needed to be temporarily reassigned. The problem was that I wanted the variable to return to its original value after the code had been executed. So I would end up assigning it to some local variable, executing the code, and then setting the global variable back to its original value held by the local variable. Here's a sample of what I mean:

        // Hold value in local variable
        double* pTemp = g_pValue;
        // Reassign it to another value (temporarily)
        g_pValue = NULL;
        // Use it in some code
        FunctionWhichUsesGlobalValue( );
        // Restore it back to what it was originally
        g_pValue = pTemp;

    As may be evident, this implementation has a couple of problems. For one, if the function throws an exception the global variable never regains its original value. Another problem is that you have to remember to restore the value. For a simple case like this one it's not difficult, but if you have multiple return points, it means having to restore the value on all of them. This results in duplicate code which is more difficult to maintain.

    So how do we solve this problem? With a class, of course! If we store the variable's original value inside a class, we can then make the destructor restore the variable's value automatically. This would give us two key advantages:

    • The variable would always regain its original value.
    • We would not have to worry about explicitly restoring it.

    Now, you probably think that the way to do this is with a template class... and you're right. A template class accomplishes this task and turns the above code into something like this:

        CTemp<double*> temp = g_pValue; 
        g_pValue = NULL; 
        FunctionWhichUsesGlobalValue( );

    That's quite an improvement, isn't it?! Well, it still has one minor problem which was also prevalent in the first example: the fact that we have to know and specify the variable's type. And while that is not too big of a deal, wouldn't it be better to not have to do it? In other words, wouldn't it be nice to just say: CTemp temp = g_pValue and the object would then somehow know that g_pValue is a pointer to a double? Well, with template member functions it can be done!

    Template member functions work just like regular template functions. The compiler generates them based on the type of the arguments on which they operate. So what is basically needed is a class with a template constructor and a way for the constructor to store the value in a generic member variable which the destructor can then properly restore. It's a bit tough to explain the exact mechanism here but I think you'll understand it once you see its implementation. The class is called CTemp and applying it to the code above would make it look like this:

        CTemp temp = g_pValue; 
        g_pValue = NULL; 
        FunctionWhichUsesGlobalValue( );

    Much nicer, don't you think?! Not only does this class do what the template version did but it does it without you having to determine and then specify the variable's type in the code. That's what I call putting the compiler to work for the programmer!

    To use the class simply download the temp.h file into your C++ project, include it where you need it, and enjoy!


    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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    About the Author

    Alvaro Mendez
    Web Developer
    United States United States
    I've done extensive work with C++, MFC, COM, and ATL on the Windows side. On the Web side, I've worked with VB, ASP, JavaScript, and COM+. I've also been involved with server-side Java, which includes JSP, Servlets, and EJB, and more recently with ASP.NET/C#.

    You may also be interested in...

    Comments and Discussions

    Questionwhy? Pin
    Jeff Reed4-Feb-00 12:14
    sussJeff Reed4-Feb-00 12:14 
    AnswerRe: why? Pin
    Alvaro Mendez8-Feb-00 9:31
    sussAlvaro Mendez8-Feb-00 9:31 
    GeneralRe: why? Pin
    Mike Junkin3-May-00 10:50
    sussMike Junkin3-May-00 10:50 

    I think Jeffs' point is most of the times you could use such a class it's usually an indication that there might be a problem with how the code is designed.

    Could be I'm not, and aparently Jeff wasn't, thinking of the right examples. But sticking to your example.

    A. globals are bad in general but having a global that is changed as a side effect to using some function is REALLY bad.

    B if it's a local variable that your are passing to a function, and you don't want that variable to change, then either the function shouldn't be changing it, or you should just be passing a copy of the variable.

    C. If a function is being passed a parameter that it modifies, it should not be because it's an un-intended side effect. It should be because part of the functions purpose is to change that parameter. Which means the results of the change are important and you wouldn't want CTemp to have any chance of rolling back the changes before you got a look at them. (Which really takes you back to B, as the most common scenerio would entail you copying the variable, sending the copy to the function and comparing the changes to the original to see what happened.)

    Anyway, I'm glad you posted the article as it raises some issues we all need to think about from time to time.

    GeneralRe: why? Pin
    Alvaro Mendez3-May-00 11:46
    sussAlvaro Mendez3-May-00 11:46 
    GeneralRe: why? Pin
    Soeren18-May-00 6:11
    sussSoeren18-May-00 6:11 
    GeneralRe: why? Pin
    Bobbby5-Feb-01 21:55
    memberBobbby5-Feb-01 21:55 
    AnswerRe: why? Pin
    Stephen Hewitt26-Jan-06 15:22
    memberStephen Hewitt26-Jan-06 15:22 

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