Preprocessor
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The preprocessor performs text replacement and preprocessing directives before a C/C++ program is compiled. Key functions of the preprocessor include #include to insert external files, #define for symbolic constants and macros, and conditional compilation (#if #ifdef #ifndef etc.) to control compilation. Other directives include #error to print error messages, #pragma for compiler-specific options, and assert to check for invalid values.
Preprocessor
- 2. Contents • The preprocessor introduction • #include • #define (Symbolic Constants) • #define (Macro) • Conditional compilation • #error and #pragma • # and ## Operators • Line Numbers • Predefined Symbolic Constants • Assertions 2
- 3. Introduction – Occurs before program compiled • Inclusion of external files (#include <stdio.h>) • Definition of symbolic constants (#define max 100) • Macros • Conditional compilation (#ifdef #elif #endif …) • Conditional execution – All directives begin with # – Directives not C++ statements • Do not end with ; – Preprocessor is the text replacement 3
- 4. #include – Puts copy of file in place of directive • Seen many times in example code – Two forms • #include <filename> – For standard library header files in SDK – Searches predestinated directories • #include "filename" – Searches in current directory – Normally used for programmer-defined files 4
- 5. #include • Example – Before preprocess – After preprocess After preprocess math.cpp void foo_1(); void foo_2() { printf(“#include me“); } // --------------------------------------- void foo() { return; } Before preprocess my_header.h void foo_1(); void foo_2() { printf(“#include me“); } math.cpp #include "my_header.h“ void foo() { return; } 5
- 6. #define Symbolic constants – Symbolic constants • Constants represented as symbols • All occurrences replaced in preprocess before compiling – Format #defineidentifier replacement-text #definePI 3.14159 – Everything to right of identifier replaces text #definePI =3.14159 Replaces PI with "=3.14159" – Cannot redefine symbolic constants 6
- 7. #define Symbolic constants #include <stdio.h> #define PI 3.14 #define LEVEL =1 void main() { float pi = PI; int lv = LEVEL; } #include <stdio.h> void main() { float pi = 3.14; int lv = =1; } 7
- 8. #define Symbolic constants • Advantages – Takes no memory • Disadvantages – Name not be seen by debugger (replacement text) – No type checking (not safe). – Using static constant instead. 8
- 9. #define Macros • Macro – Operation specified in #define – Intended for legacy C programs – Macro without arguments • Treated like a symbolic constant – Macro with arguments • Arguments substituted for replacement text • Macro expanded – Performs a text substitution • No data type checking 9
- 10. #define Macros • Example #define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) ) area = CIRCLE_AREA( 4 ); becomes area = ( 3.14159 * ( 4 ) * ( 4 ) ); • Use parentheses – Without them, #define CIRCLE_AREA( x ) PI * x * x area = CIRCLE_AREA( c + 2 ); becomes area = 3.14159 * c + 2 * c + 2; which evaluates incorrectly 10
- 11. #define Macros • Multiple arguments #define RECTANGLE_AREA( x, y ) ( ( x ) * ( y ) ) rectArea = RECTANGLE_AREA( a + 4, b + 7 ); becomes rectArea = ( ( a + 4 ) * ( b + 7 ) ); • #undef – Undefines symbolic constant or macro – Can later be redefined 11
- 12. *Conditional compilation • Control preprocessor directives and compilation – Cannot evaluate cast expressions, sizeof, enumeration constants • Structure similar to if #if !defined( NULL ) #define NULL 0 #endif – Determines if symbolic constant NULL defined – If NULL defined, • defined( NULL ) evaluates to 1 • #define statement skipped – Otherwise • #define statement used – Every #if ends with #endif 12
- 13. *Conditional compilation • Can use else #else #elif is "else if“ • Abbreviations #ifdef short for #if defined(name) #ifndef short for #if !defined(name) 13
- 14. *Conditional compilation • "Comment out" code – Cannot use /* ... */ with C-style comments • Cannot nest /* */ – Instead, use #if 0 code commented out #endif – To enable code, change 0 to 1 14
- 15. *Conditional compilation • Debugging #define DEBUG 1 #ifdef DEBUG cerr << "Variable x = " << x << endl; #endif – Defining DEBUG enables code – After code corrected • Remove #define statement • Debugging statements are now ignored 15
- 16. #error and #pragma • #error tokens – Prints implementation-dependent message – Tokens are groups of characters separated by spaces • #error 1 - Out of range error has 6 tokens – Compilation may stop (depends on compiler) • #pragma tokens – Actions depend on compiler – May use compiler-specific options – Unrecognized #pragmas are ignored 16
- 17. # and ## operators • # operator – Replacement text token converted to string with quotes #define HELLO( x ) cout << "Hello, " #x << endl; – HELLO( JOHN ) becomes • cout << "Hello, " "John" << endl; • Same as cout << "Hello, John" << endl; • ## operator – Concatenates two tokens #define TOKENCONCAT( x, y ) x ## y – TOKENCONCAT( O, K ) becomes • OK 17
- 18. Line numbers • #line – Renumbers subsequent code lines, starting with integer • #line 100 – File name can be included – #line 100 "file1.cpp" • Next source code line is numbered 100 • For error purposes, file name is "file1.cpp" • Can make syntax errors more meaningful • Line numbers do not appear in source file 18
- 19. Predefined symbolic constants Symbolic constant Description __LINE__ The line number of the current source code line (an integer constant). __FILE__ The presumed name of the source file (a string). __DATE__ The date the source file is compiled (a string of the form "Mmm dd yyyy" such as "Jan 19 2001"). __TIME__ The time the source file is compiled (a string literal of the form "hh:mm:ss"). Five predefined symbolic constants Cannot be used in #define or #undef 19
- 20. Assertions • assert is a macro – Header <cassert> – Tests value of an expression • If 0 (false) prints error message, calls abort – Terminates program, prints line number and file – Good for checking for illegal values • If 1 (true), program continues as normal – assert( x <= 10 ); • To remove assert statements – No need to delete them manually – #define NDEBUG • All subsequent assert statements ignored 20