Macros

A macro is a label defined in the source code that is replaced by its value by the preprocessor before compilation. Macros are initialized with the #define preprocessor command and can be undefined with the #undef command.

There are two types of macros: object-like macros and function-like macros.

Object-Like Macros

These macros are replaced by their value in the source code before compilation. Their primary purpose is to define constants to be used in the code.

Note: Macro definitions are not followed by a semicolon ;.

Example

In the following example, PI is defined as an object-like macro:

#include <iostream>

using namespace std;

#define PI 3.1416

int main() {
  float radius = 3;
  float area;

  area = PI * radius * radius;

  cout << "Area is " << area;

  return 0;
}

This results in the output:

Area is 28.2744

Function-Like Macros

These macros behave like functions, in that they take arguments that are used in the replaced code.

Note: When defining a function-like macro, there cannot be a space between the macro name and the opening parenthesis.

Example

In the following example, AREA is defined as a function-like macro. Note that other macros can be used in defining a subsequent macro. The inner macro is replaced by its value before the outer macro is replaced.

#include <iostream>

using namespace std;

#define PI 3.1416
#define AREA(r) r * r * PI

int main() {
  float radius = 5;
  float result;

  result = AREA(radius);

  cout << "Area is " << result;

  return 0;
}

This results in the output:

Area is 78.54

Predefined Macros

C++ has a number of predefined macros, including the following:

• __DATE__: Current date formatted as MMM DD YYYY.
• __TIME__: Current time formatted as HH:MM:SS.
• __FILE__: Current filename.
• __LINE__: Current line number.

Example

The following example uses the predefined macros mentioned above:

#include <iostream>

using namespace std;

int main() {
  char file[] = __FILE__;
  char date[] = __DATE__;
  char time[] = __TIME__;
  int line = __LINE__;

  cout << "File name: " << file << "\n";
  cout << "Date: " << date << "\n";
  cout << "Time: " << time << "\n";
  cout << "Line number: " << line << "\n";
}

This results in the output:

File name: main.cpp
Date: Jun 25 2022
Time: 14:17:50
Line number: 9

Undefining a Macro

Once defined, a macro can be undefined with the #undef command. Using the macro after that point will result in a compile error.

Example

#include <iostream>

using namespace std;

#define TEST 1

int main() {
  #ifdef TEST
    cout << "TEST defined\n";
  #else
    cout << "TEST undefined\n";
  #endif

  #undef TEST

  #ifdef TEST
    cout << "TEST defined\n";
  #else
    cout << "TEST undefined\n";
  #endif
}

This results in the output:

TEST defined
TEST undefined