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[SOLVED] [C/C++] The use of #define, only preprocessor?

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David_

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Hello.

While trying to learn something about macros I found that there are a directive #undef, and I wonder if the following:

Code: 
#ifndef VALUE
#define VALUE 10
...
if(something)
{
    #ifdef VALUE
    #undef VALUE
    #define VALUE 20
}

Is this exclusively used as a preprocessor directive or can one use such a code during runtime to define things depending on button presses as a example?

Regards
 

can use #define in other than preprocessing section.
 

The preprocessing directives need not be defined in the top section of the code. They can used even inside your functions, but they wont be executed at 'runtime', the conditional preprocessing directives will be evaluated during compilation and be placed as some final value in compiled code.


David_ said:
Hello.

While trying to learn something about macros I found that there are a directive #undef, and I wonder if the following:

Code: 
#ifndef VALUE
#define VALUE 10
...
if(something)
{
    #ifdef VALUE
    #undef VALUE
    #define VALUE 20
}

Is this exclusively used as a preprocessor directive or can one use such a code during runtime to define things depending on button presses as a example?

Regards
Click to expand...
 

can one use such a code during runtime to define things depending on button presses as a example?
Click to expand...

NO, pre-processor works before compiling result, so
you only can prepare some conditional compiling option ..

to modify the behavior or your application during runtime, .. just code it , with your external events ..
Code: 
 if( BP1==1) 
{
 do something
 use UART1 ...
  }
else
 {
  do other job
  use LCD ..
 }
 

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