Understanding Preprocessor Directives in Windows Programming: #define, #include, #if, and , Slides of Windows Programming

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Windows Programming

Lecture 05

Preprocessor

Preprocessor Directives

Preprocessor directives are instructions for

compiler.

Examples of Preprocessor

Directives

#define #error #undef #include

#if #else #elif #endif

#ifdef #line #ifndef #pragma

#define ARRAY_SIZE 10

Before Preprocessing

char array[ARRAY_SIZE];

After Preprocessing

char array[10];

identifier is not replaced if it appears

• in a comment
• within a string, or
• as part of a longer identifier

#define directive

#define WIDTH 10

#define HEIGHT WIDTH+

HEIGHT is replaced with^ WIDTH+25 is replaced with 10+

#define directive

identifier remains defined and can be tested

• It can be checked using #if and #ifdef

directives.

• #if defined is same as #ifdef.

#if, #elif, #else

#elif is same as #else #if

Preprocessor Operators

The defined is a preprocessor operator.

#if defined MAX

is equivalent to

#ifdef MAX

Preprocessor operator

Preprocessor Directive

#error directive

• The directive #error causes the preprocessor to report a fatal error. The rest of the line that follows #error is used as the error message. The line must consist of complete tokens.
• You would use #error inside of a conditional statment

that detects a combination of parameters which you know the program does not properly support.

#error directive

#if !defined(__cplusplus)

#error Must use C++ language

#endif

#undef Directive

Once an identifier has been undefined, that

identifier may be redefined by a subsequent

#define directive. The new definition need not

have any resemblance to the old definition.

#undef Directive

#define SPEED ………… some code here…………… … … … … … … … #undef SPEED ifndef(SPEED) #error Speed not defined #endif