Editing C preprocessor (section)

===Operators===
The preprocessor is capable of interpreting operators and evaluating very basic expressions, such as integer constants, arithmetic operators, comparison operators, logical operators, bitwise operations, the <code>defined</code> operator, and the <code>#</code> stringificafion operator. This allows the preprocessor to make evaluations such as:
<syntaxhighlight lang="cpp">
#if X == 10 // if X equals 10, the preprocessor sees #if 10 == 10
</syntaxhighlight>

====Defined operator====
While the '''defined operator''', denoted by <code>defined</code> is not a directive in its own right, if it is read within a directive, it is interpreted by the preprocessor and determines whether a macro has been defined. 

The following are both accepted ways of invoking the <code>defined</code> operator.
<syntaxhighlight lang="cpp">
#if defined(MY_MACRO)
#if defined MY_MACRO
</syntaxhighlight>

====Token stringification operator====
The '''stringification operator''' (a.k.a. stringizing operator), denoted by <code>#</code> converts a token into a [[string literal]], escaping any quotes or backslashes as needed. For definition:

<syntaxhighlight lang="cpp">
#define str(s) #s
</syntaxhighlight>

<code>str(\n)</code> expands to <code>"\n"</code> and <code>str(p = "foo\n";)</code> expands to <code>"p = \"foo\\n\";"</code>.

If stringification of the expansion of a macro argument is desired, two levels of macros must be used. For definition:

<syntaxhighlight lang="cpp">
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
</syntaxhighlight>

<code>str(foo)</code> expands to "foo" and <code>xstr(foo)</code> expands to "4".

A macro argument cannot be combined with additional text and then stringified. However, a series of adjacent string literals and stringified arguments, also string literals, are concatenated by the C compiler.

====Token concatenation====
The '''token pasting operator''', denoted by <code>##</code>, concatenates two tokens into one. For definition:

<syntaxhighlight lang="cpp">
#define DECLARE_STRUCT_TYPE(name) typedef struct name##_s name##_t
</syntaxhighlight>

<code>DECLARE_STRUCT_TYPE(g_object)</code> expands to <code>typedef struct g_object_s g_object_t</code>.