Global vs local variables

[rajeevktripathi]

Hi All I have a confusion related to global and local variables... why global variables are initialized to zero whereas local variables are not... What is the reason behind this. Please suggest me the answer. Thanks & Regards

[Waldermort]

Global variables can be used within any function from anywhere within your code, local variables can only be used within scope that they were created ( usualy a single function ). It's good practice and reduces many bugs if you initialize all your variables when you create them, regardless of what the books tell you.

[rajeevktripathi]

thanks WalderMort it is fine that we should initialize each var while creating it, but wondering that why global variables are initialized to zero by compiler whereas local variables are not.. Please help to understand the logic Thanks & Regards

[Waldermort]

In a debug build, variables are initialized, but you will find that within a release build, they are not. This is one of the main reasons why people complain that their release build doesn't work. Take the following example: bool bIsEqual; if ( ... ) bIsEqual = true; if ( bIsEqual ) // Is this true or false? in debug it was initialized to false ....

[rajeevktripathi]

Hi could not understand your example I wrote an example As follows: int pg; int _tmain(int argc, _TCHAR* argv[]) { int pl; int s; printf("Global %d \n", pg); printf("Local %d \n" , pl); scanf("%d", &s); return 0; } and while printing pl prompts an error message "Run-Time Check Failure #3 - The variable 'pl' is being used without being defined." any comments on this....

[Naveen]

rajeevktripathi wrote:

why global variables are initialized to zero whereas local variables are not...

During the CRT initialization routine ( wWinMainCRTStartup function ), there is special code for intializing the c objects and c++ objects. You can find a function called _initterm() called from the wWinMainCRTStartup. That function does all this.

nave [OpenedFileFinder]

[jhwurmbach]

rajeevktripathi wrote:

"Run-Time Check Failure #3 - The variable 'pl' is being used without being defined."

At compile time, you have got a "warning C4700: local variable 'pl' used without having been initialized". As you have enabled "Runtime checks" (probably a Debug-build), the runtime complains that you use this variable without prior assignment to it.

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[David Crow]

rajeevktripathi wrote:

why global variables are initialized to zero whereas local variables are not...

You'll likely see a difference in initialization between release and debug compiles of your program. In debug mode, since the frame pointer is always pushed onto the stack at routine entry, variables are almost always assigned locations on the stack (e.g., 0). In the release mode, optimizations of the compiler (and other factors) may detect that the frame pointer is not needed, so the frame pointer is not pushed onto the stack, thus no initial value.

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