Preprocessor Directives

When Pike executes a program file, the source code is first run through the preprocessor, The preprocessor performs some textual transformations on the source code. By "textual transformations" we mean that the preprocessor doesn't know a lot about Pike, and just looks at the source code as text, where it for example can replace some of the words.

We recommend that you don't use the preprocessor at all, unless you have to. Most of what the preprocessor can do, can be done in better ways in Pike.

The preprocessor is controlled by preprocessor directives. A preprocessor directive must be written on its own line, and starts with a hash character (#). This is an example of a preprocessor directive:

#define MAX_TEMP 100

This directive defines the macro MAX_TEMP. The preprocessor will now replace the word MAX_TEMP with 100 in the rest of the file, before translating and executing the source code.

Try to avoid this. Pike will replace every occurrence of MAX_TEMP in the rest of the file with 100, and this can cause all sorts of trouble. It is better to use a real constant:

constant MAX_TEMP = 100;

Preprocessor macros can have arguments:

#define square(x) x * x

This directive will cause Pike to replace every occurrence of "square(expression)" with "expression * expression". But note that this is just a textual transformation! As an example of what this means, the statement

i = square(2 + 3);

which we expect to assign 25 to the variable i, will be translated to

i = 2 + 3 * 2 + 3;

where multiplication, as always, has higher precedence than addition. The result is that a value of 11 will be assigned to i. Macros have several other problems similar to this one, so whenever you can you should use a method instead:

int square(int x) { return x * x; }

Another unfortunately common preprocessor directive is the include directive:

#include "somefile.pike"

Before the source code is translated, this directive will be replaced by the contents of the file "somefile.pike". By including another Pike file, we can implement a primitive form of inheritance. If that is what you are doing, it is much better to use the real inheritance mechanism:

inherit .somefile;

One thing that you can use the preprocessor for is to tell Pike which character set you are using in the Pike source code:

#charset <charset-name>

The default character set is iso8859-1, which is the right one for English and most Western European languages. Some other character sets are utf-8 and iso2022.

Magic Preprocessor Constants

The preprocessor also expands a few specific keywords to pike constants (C programmers will feel right at home); a __LINE__ in your code will be translated to an integer pointing out at which line the keyword was found. __DATE__ will be expanded to a "Mon day Year" string constant, e g "May 2 2000" with the date when the file was compiled (or, more to the point, when the preprocessor replaced the entity). Similarly, __TIME__ turns into a 24-hour "HH:MM:SS" time string constant, e g "19:17:21".