#if

The preprocessor enables the conditional copying of its input. At the start, its default mode is copy, meaning that characters in the input are copied to the output. Conditional directives enable another mode: skip, by means of which the following characters are discarded, and only newline characters are copied.

Conditional directives follow the familiar syntax of some of their cousins in programming languages. At the very least,

1. they start with the #if directive, followed by a Boolean expression as argument,

2. and they are closed by #endif.

It is also possible to use

• one #else directive before #endif;

• a series of #elif directive after #if (as a short-hand for a #else immediately followed by an #if, except that only one #endif will close the conditional).

A trivial example would be:

#if false

This is NOT copied to the output, except the newline character

#endif

where false is a predefined symbol acting like a Boolean value. The output is

# 1 "Tests/test.txt"

Note what looks like an anonymous preprocessing directive # 1 "Tests/test.txt". We will explain its meaning when presenting The Inclusion Directive. (Remark: cpp would not output blank lines followed by the end of the file.) Their use is clearer if we add text before and after the conditional, like so:

---

#if false

A

#endif

---

whose preprocessed output is then

# 1 "Tests/test.txt"

---

---

Here is how to use the #else directive:

#if false

This is NOT copied to the output, except the newline character.

#else

This IS copied to the output.

#endif