Compiler instructions to be carried out before the actual compilation.
Always begins with a hash symbol #
Not terminated with a semi-colon ;
Used when compiling the program.
The most common preprocessor directives are: #include and #define
| #define | Defines a preprocessor macro |
| #include | Substitues a preprocessor macro |
| #undef | Undefines a macro |
| #ifdef | Returns TRUE if macro is defined |
| #ifndef | Returns TRUE if macro is not defined |
| #if | Tests if a compile time condition is TRUE |
| #else | the alternative for #if |
| #elif | #else and #if in one statement |
| #endif | Ends preprocessor conditional |
| #error | Prints error message on stderr |
Conditional Compilation can be carried out by utilising the various conditional preprocessor directives stated above. This example defines (or in this case, sets) a condition to be checked upon:
#include <iostream>
using namespace std;
#define DEBUG
int main () {
#ifdef DEBUG
cerr << "Debug message goes here." << endl ;
#endif
cout << "This simply shows what happens when the above DEBUG" << \
" (not a keyword) is defined/set." << endl ;
#ifdef DEBUG
cerr << "Debug message goes here." << endl ;
#endif
cout << "Comment out the above #define preprocessor directive to 'turn off' " << \
"debugging comments." << endl ;
#ifdef DEBUG
cerr << "Debug message goes here." << endl ;
#endif
return 0;
}
| Debug message goes here. Debug message goes here. Debug message goes here. This simply shows what happens when the above DEBUG (not a keyword) is defined/set. Comment out the above #define preprocessor directive to 'turn off' debugging comments. |
The # preprocessor operator can be used to convert specified text into a quoted string:
#include <iostream>
using namespace std;
#define STRINGTHIS( a ) #a //note, using same argument after #
int main () {
cout << STRINGTHIS(Hello World!) << endl ;
return 0;
}
| Hello World! |