//*
[code here]
//*/

// in the following , foo() is active:
/**/ foo(); /*/ bar(); /**/

// now bar() is active:
/*/ foo(); /*/ bar(); /**/

#ifdef ENABLE_TESTS
// code that you want to run ONLY during tests
#endif

$ gcc -DENABLE_TESTS source.c

#if 0
...disabled code here
#endif

//* <-- remove the first slash
[code block 1]
/*/
[code block 2]
//*/

[code block 1]

if (0)
{
    [code block 1]
}
else
{
    [code block 2]
}

#if 0
    //code goes here
#endif

bool debugging = false;

// banana banana banana

if (debugging)
{
    // do a bunch of stuff here
}

void doNothing(){}
#define DO_IF(flag, code) flag ? code : doNothing();

DO_IF(collectStats, recordStats());
DO_IF(collectStats, store(pullStat()));

#define DO_IF(flag,code) if( flag ) { code; }

#define COMPILE

#ifdef COMPILE

//code to comment begins
cout<<"ha ha ha"<<endl;
//code to comment ends

#endif

1 ? foo() : bar();

if(isMode1)
{
    //Code for mode1
}
else
{
    //Code for other modes
}

#define CRAZY_EXPERIMENT

#ifdef TEST
#include "Test.h"
#elif ANOTHER_TEST
#include "AnotherTest.h"
#elif CRAZY_EXPERIMENT
#include "CrazyExperiment.h"
#else

int main(int argc, int * argv[]){
    runTheProgramLikeNormal();
}

#endif

#ifdef _DEBUG
    #define IF_DEBUG(x) if(x)
#else
    #define IF_DEBUG(x) if(false)
#endif

#include "debug.hpp"

int a,b, ... ,z;

...

IF_DEBUG(... regular_expression_here_with_a_b_z ...) {
    // set of asserts
    assert(... a ...);
    assert(... b ...);
    ...
    assert(... z ...);
}

IF_DEBUG(... regular_expression_here_with_a_b_z ...) {
    // set of asserts
    assert(... a ...);
    assert(... b ...);
    ...
    assert(... z ...);
}
else {
    (void)a;
    (void)b;
    ....
    (void)z;
}