C++ Conditional Compilation

Table of Contents

Conditional Compilation

Allows fine-tuning over what to compile and what not. Utilizes:

• ifdef
• ifndef
• endif ifdef: checks a condition if true to continue compiling before the next endif ifndef: checks a condition if false to continue compiling before endif

ifdef and ifndef can both be replaced with if defined(arg) and if !defined(arg) respectively and both can be used as Preprocessor Directives

ifdef Example

#include <iostream>
 
#define PRINT_JOE
 
int main()
{
#ifdef PRINT_JOE
    std::cout << "Joe\n"; // will be compiled since PRINT_JOE is defined
#endif
 
#ifdef PRINT_BOB
    std::cout << "Bob\n"; // will be excluded since PRINT_BOB is not defined
#endif
 
    return 0;
}

ifndef Example

#include <iostream>
 
int main()
{
#ifndef PRINT_BOB
    std::cout << "Bob\n";
#endif
 
    return 0;
}

if 0

used as comments to block lines of code from compilation

#include <iostream>
 
int main()
{
    std::cout << "Joe\n";
 
#if 0 // Don't compile anything starting here
    std::cout << "Bob\n";
    std::cout << "Steve\n";
#endif // until this point
 
    return 0;
}

temporarily ‘remove’ the comment by changing it to if 1