C++ Tutorial/Development/ifdef

define macro

<source lang="cpp">#define DemoVersion

#define PC 5
#include <iostream>

int main()
{
    std::cout << "Checking on the definitions of DemoVersion,PC and MAC...\n";

#ifdef DemoVersion
    std::cout << "DemoVersion defined.\n";
#else // DemoVersion
    std::cout << "DemoVersion not defined.\n";
#endif // DemoVersion

#ifndef PC
    std::cout << "PC not defined!\n";
#else // PC
    std::cout << "PC defined as: " << PC << std::endl;
#endif // PC

#ifdef MAC
    std::cout << "MAC defined!\n";
#else // MAC
    std::cout << "MAC was not defined.\n";
#endif // MAC

    std::cout << "Done.\n";
    return 0;
}</source>

Checking on the definitions of DemoVersion,PC and MAC...
DemoVersion defined.
PC defined as: 5
MAC was not defined.
Done.

Using #undef.

<source lang="cpp">#define DemoVersion

1. define DOS_VERSION 5
2. include <iostream>

using namespace std; int main() {

1. ifdef DemoVersion

  cout << "DemoVersion defined.\n";

1. else

  cout << "DemoVersion not defined.\n";

1. endif
2. ifndef DOS_VERSION

  cout << "DOS_VERSION not defined!\n";

1. else

  cout << "DOS_VERSION defined as: " << DOS_VERSION << endl;

1. endif
2. ifdef WINDOWS_VERSION

  cout << "WINDOWS_VERSION defined!\n";

1. else

  cout << "WINDOWS_VERSION was not defined.\n";

1. endif
2. undef DOS_VERSION
3. ifdef DemoVersion

  cout << "DemoVersion defined.\n";

1. else

  cout << "DemoVersion not defined.\n";

1. endif
2. ifndef DOS_VERSION

  cout << "DOS_VERSION not defined!\n";

1. else

  cout << "DOS_VERSION defined as: " << DOS_VERSION << endl;

1. endif
2. ifndef WINDOWS_VERSION

  cout << "WINDOWS_VERSION defined!\n";

1. else

  cout << "WINDOWS_VERSION was not defined.\n";

1. endif

cout << "Done.\n";
return 0;

}</source>