CS 161 Introduction to Programming and Problem Solving Chapter 13 C++ Preprocessor Herbert G. Mayer, PSU Status 10/8/2014 Initial content copied verbatim from ECE 103 material by prof. Phillip PSU

Syllabus Preprocessor Directives Include Files Macros Named Constants Examples

2 Preprocessor Directives Before a C++ program is compiled, the source code file is first handled by a preprocessor prog_1.c prog_2.c prog_3.c etc. Source files Compiler Preprocessor prog_1.h prog_2.h prog_3.h etc. User Header files stdio.h stdlib.h math.h etc. Library Header files Assembler To Linker

3 The C++ preprocessor does this:  Strips comments from the source code  Acts on preprocessor directives The preprocessor does not compile code! Preprocessor directive → a special command recognized only by the preprocessor  # symbol prefixes each directive Directives are used for:  Modifying the compilation process  Creating macro definitions

4 Common directives: #include f:Inserts the contents of a specified text file into the source code #define m:Defines a macro #undef m:Cancels a previously defined macro #if expr:Begins a conditional block → expr tested #ifdef m:Tests whether a macro is defined #ifndef m:Tests whether a macro is not yet defined #else:Alternative branch #elif expr:Else if → expr tested #endif:Ends a conditional block

5 Include Files C++ provides access to common functions (e.g., I/O, math routines) via external libraries Each library has a header file that may contain prototypes, macros, and definitions A header file is a text file with a.h extension Before accessing a library’s functions, use the #include directive to load its header file #include directives are typically placed at the beginning of the source file

6 Syntax: #include Angle bracket form → searches for a header file in the system include directory #include "filename" Quoted form → searches for a header file starting with the current working directory

7 Example: /* Loads system header for stdio library */ #include /* Loads system header for math library */ #include /* Loads user header file */ #include "mystuff.h" /* Loads user header file */ #include "../LIB/mylib.h"

8 Macros A preprocessor macro is used for:  Defining a symbolic name for a constant value  Defining a terse alias for something else To define a macro, use this directive: #define mname mtext mname → macro name, formal parameters allowed mtext → substitution text At least one whitespace

9 The preprocessor expands macros in the C code Wherever it sees mname in the source file, it substitutes mtext in its place A macro definition can contain previously defined macros Macros are not at all the same as C variables! No storage is created in memory; macros only exist at compile-time An old C programming convention rules that macro names be fully capitalized to differentiate them from C variable names; this is NOT a language rule, hence NOT enforced by the C compiler!

10 Example: /* Pay rate in $ per hour */ /* Time in hours per week */ #include #define RATE #define HRS_WK1 40 int main (void) { // main float rate2 = 22.50; float hrs_wk2 = 35; float pay1, pay2; /* Weekly pay in $ */ pay1 = RATE1 * HRS_WK1; pay2 = rate2 * hrs_wk2; printf("RATE1=%f\n",RATE1); return 0; } //end main [stdio.h definitions] [are inserted here ] int main (void) { // main float rate2 = 22.50; float hrs_wk2 = 35; float pay1, pay2; pay1 = 9.25 * 40; pay2 = rate2 * hrs_wk2; printf("RATE1=%f\n",9.25); return 0; } //end main Original source fileAfter macro expansion

11 Macro expansion is very literal. Be careful of unintended side-effects Example: #define A 5 #define B A + 1 #define FACTOR1 A * B/ #define FACTOR2 (A) * (B)/ #define Tip_Rate 0.15 #define UINT unsigned int #define PRINTBLANK printf("\n")

12 Commenting Out Code By Using #if … #endif This is illegal in C90. Comments cannot be nested within other comments. /* x = r * cos(a); /* x component */ y = r * sin(a); /* y component */ */ This is legal in C90. The code within the #if 0 … #endif is removed from the source before it ever reaches the compiler. #if 0 x = r * cos(a); /* x component */ y = r * sin(a); /* y component */ #endif

13 Named Constants magic number → a non-obvious literal number or character that is directly embedded in a statement Avoid magic numbers. Use named constants instead! Example: /* Newton's equation of motion */ y = –0.5 * 9.81*t*t *t + 2.5; What do these numbers mean? 9.81 = Earth’s gravitational acceleration g (m/s 2 ) 75.2 = Initial velocity v y0 (m/s) in y -axis direction 2.5 = Initial position y 0 (m) in y -axis direction

14 Example: #include #define G_EARTH 9.81 // m/s^2 #define VELOCITY_Y // m/s #define POSITION_Y0 2.5 // m int main (void) { // main double t; // Time (s) double y; // Height (m) at time t printf( "Enter time (s): ” ); scanf( "%lf", &t ); y = -0.5 * G_EARTH * t * t + VELOCITY_Y0 * t + POSITION_Y0; printf( "y(%.2f s) = %.2f m\n", t, y ); return 0; } //end main

15 Question: Can I just use constants instead? Yes, you could define something like this: static const double g_earth = 9.81; Comparison of #define versus const #defineconst variable Cannot create a pointer to a macroCan create a pointer to a const variable Ignores scopeRespects scope and is type-safe No symbol in the debuggerHas a symbol in the debugger Can be used to set the size of an arrayCannot be used as array dimension Can be used in a switch case valueCannot be used in a switch case