Source and Header Files

In computer science, header files are a feature of some programming languages (most famously C and C++) that allows programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers. Programmers who wish to declare standardized identifiers in more than one source file can place such identifiers in a single header file, which other code can then include whenever the header contents are required. The C standard library and C++ standard library traditionally declare their standard functions in header files.

Admin wrote:hey can you explain to me why people sometimes write #ifndef, #define etc in the start of their files?

After reading about it I still don't fully understand

Wikipedia wrote:
The C preprocessor (cpp) is the preprocessor for the C programming language. In many C implementations, it is a separate program invoked by the compiler as the first part of translation. The preprocessor handles directives for source file inclusion (#include), macro definitions (#define), and conditional inclusion (#if). The language of preprocessor directives is agnostic to the grammar of C, so the C preprocessor can also be used independently to process other types of files.

The transformations it makes on its input form the first four of C's so-called Phases of Translation. Though an implementation may choose to perform some or all phases simultaneously, it must behave as if it performed them one-by-one in order.

Wikipedia wrote:
Conditional compilation

The #if, #ifdef, #ifndef, #else, #elif and #endif directives can be used for conditional compilation.

#ifdef _WIN32 // _WIN32 is defined by all Windows 32 compilers, but not by others.
#include <windows.h>
#else
#include <unistd.h>
#endif

#if VERBOSE >= 2
print("trace message");
#endif

Some compilers targeting Windows define WIN32, but all should define _WIN32[1] This allows code, including preprocessor commands, to compile only when targeting windows systems. Alternatively, the macro WIN32 could be defined implicitly by the compiler, or specified on the compiler's command line, perhaps to control compilation of the program from a makefile.

The example code tests if a macro _WIN32 is defined. If it is, the file <windows.h> is included, otherwise <unistd.h>.

A more complex example might be something like

#if !defined(WIN32) || defined(__MINGW32__) // non windows or mingw compiler
...
#endif

Macro definition and expansion

There are two types of macros, object-like and function-like. Object-like macros do not take parameters; function-like macros do. The generic syntax for declaring an identifier as a macro of each type is, respectively,

#define <identifier> <replacement token list>
#define <identifier>(<parameter list>) <replacement token list>

Note that the function-like macro declaration must not have any whitespace between the identifier and the first, opening, parenthesis. If whitespace is present, the macro will be interpreted as object-like with everything starting from the first parenthesis added to the token list.

Whenever the identifier appears in the source code it is replaced with the replacement token list, which can be empty. For an identifier declared to be a function-like macro, it is only replaced when the following token is also a left parenthesis that begins the argument list of the macro invocation. The exact procedure followed for expansion of function-like macros with arguments is subtle.

Object-like macros were conventionally used as part of good programming practice to create symbolic names for constants, e.g.

#define PI 3.14159

... instead of hard-coding those numbers throughout one's code. An alternative in both C and C++ is to apply the const qualifier to a global variable.

An example of a function-like macro is:

#define RADTODEG(x) ((x) * 57.29578)

This defines a radians to degrees conversion which can be written subsequently, e.g. RADTODEG(34) or RADTODEG (34). This is expanded in-place, so the caller does not need to litter copies of the multiplication constant all over his code. The macro here is written as all uppercase to emphasize that it is a macro, not a compiled function.

Random number (100,300) : 126

Admin wrote: I see. Also paul, you're an asshole.


Also I didn't steal the gravity code dumbass.

takuroindigo wrote:I missed keeping track of mine at the beginnning... I guess it won't do much good now...

Admin wrote:would it be the rpg game you showed me at the beginning of the year? with the hatchet