define macro
#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;
}Checking on the definitions of DemoVersion,PC and MAC...
DemoVersion defined.
PC defined as: 5
MAC was not defined.
Done.
Using #undef.
#define DemoVersion
#define DOS_VERSION 5
#include <iostream>
using namespace std;
int main()
{
#ifdef DemoVersion
cout << "DemoVersion defined.\n";
#else
cout << "DemoVersion not defined.\n";
#endif
#ifndef DOS_VERSION
cout << "DOS_VERSION not defined!\n";
#else
cout << "DOS_VERSION defined as: " << DOS_VERSION << endl;
#endif
#ifdef WINDOWS_VERSION
cout << "WINDOWS_VERSION defined!\n";
#else
cout << "WINDOWS_VERSION was not defined.\n";
#endif
#undef DOS_VERSION
#ifdef DemoVersion
cout << "DemoVersion defined.\n";
#else
cout << "DemoVersion not defined.\n";
#endif
#ifndef DOS_VERSION
cout << "DOS_VERSION not defined!\n";
#else
cout << "DOS_VERSION defined as: " << DOS_VERSION << endl;
#endif
#ifndef WINDOWS_VERSION
cout << "WINDOWS_VERSION defined!\n";
#else
cout << "WINDOWS_VERSION was not defined.\n";
#endif
cout << "Done.\n";
return 0;
}