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A compiler directive is an instruction to the compiler to complete a task before formally starting to compile the program, thus they are sometimes called pre-processor directives. Among other items, during the pre-processor step the compiler is looking for compiler directives and processes them as they are encountered. After completing the tasks as directed, the compiler proceeds to its second step where it checks for syntax errors (violations of the rules of the language) and converts the source code into an object code that contains machine language instructions, a data area, and a list of items to be resolved when he object file is linked to other object files.
Within C++ the pound symbol or # as the first character of a line indicates that the next word is a directive (or command word) to be evaluated. The two most common compiler directives are:
In the following example the include directive is inserting a file that contains code from the Input-Output Stream library. This file contains necessary code to use cout and cin for sending data to the monitor or getting data from the keyboard.
#include<iostream>
In the next example the define directive is being used to handle a constant (called a defined constant).
#define PI 3.14159
....Later on in the program when it encounters PI....it will replace or substitute PI with the value 3.14159
....For example:area_circle = radius * radius * PI;
would become:area_circle = radius * radius * 3.14159;
Of note, compiler directives in C++ do not have a semi-colon after them. Within C++ programming instructions or statements end with a semi-colon, but not compiler directives.
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