What does mean by "inactive" preprocessor block in C? - c++

Question

I found some lines of code, those are dimmed in my preprocessor block of source code in C. My compiler, MS Visual Studio, naming it "inactive preprocessor block". What does this mean, will my compile do not consider these lines of code,
and how to make it active block?

Answer 1

An inactive preprocessor block is a block of code that is deactivated because of a preprocessor directive. The simplest example is:
#if 0
//everytyhing here is inactive and will be ignored during compilation
#endif
A more common example would be
#ifdef SOME_VAR
// code
#else
// other code
#endif
In this case either the first or the second block of code will be inactive depending on whether SOME_VAR is defined.

Answer 2

Please check this hypothetical example created to elaborate your question.
#include <iostream>
#include <windef.h>
#define _WIN32
int add(int n1, int n2){return n1 + n2;}
LONGLONG add(LONGLONG n1, LONGLONG n2){return n1 + n2;}
int _tmain(int argc, _TCHAR* argv[])
{
#ifdef _WIN32
int val = add(10, 12);
#else
LONGLONG = add(100L, 120L);//Inactive code
#endif // _WIN32
return 0;
}
You can see as _WIN32 is defined the code in #else pre-processor directive is disabled and would not be compiled. You can undefine _WIN32 to see reverse in action. See the screen shot of MS Visual Studio attached. The line in red is disabled code.
Hope this would help.

Answer 3

The preprocessor is one the earliest stages of a programs translation. It can modify the source of the program before the compilation stage begins. That way you can configure the source to build differently, depending on various constraints.
Uses of preprocessor condition blocks include:
Completely commenting out code:
#if 0
// The code here is never compiled. It's "commented" away
#endif
Provide different implementations based on various constraints, like platfrom
#if defined(WIN32)
//Implement widget with Win32Api
#elif defined(MOTIF)
// Implement widget with Motif framework
#else
#error "Unknown platform"
#endif
Have a macro like assert behave in different ways.
Make sure a useful abstraction is defined appropriately:
#if PLATFORM_A
typedef long int32_t;
#elif PLATFORM_B
typedef int int32_t;