Editing C preprocessor (section)

==Non-standard features ==

=== Pragma ===
The <code>#pragma</code> directive is defined by standard languages, but with little or no requirements for syntax after its name so that compilers are free to define subsequent syntax and associated behavior. For instance, a pragma is often used to allow suppression of error messages, manage heap and stack debugging and so on.

C99 introduced a few standard pragmas, taking the form <code>#pragma STDC ...</code>, which are used to control the floating-point implementation. The alternative, macro-like form {{code|_Pragma(...)}} was also added.

=== Trigraphs ===
Many implementations do not support trigraphs or do not replace them by default.

=== Assertion ===
Some [[Unix]] preprocessors provided an [[assertion (computing)|assertion]] feature {{endash}} which has little similarity to standard library assertions.<ref>[https://gcc.gnu.org/onlinedocs/cpp/Obsolete-Features.html GCC Obsolete features]</ref>

=== Include next ===
GCC provides <code>#include_next</code> for chaining headers of the same name.<ref>{{Cite web|url=https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html|title = Wrapper Headers (The C Preprocessor)}}</ref>

=== Import ===
Unlike C and C++, Objective-C includes an <code>#import</code> directive that is like <code>#include</code> but results in a file being included only once {{endash}} eliminating the need for include guards and [[pragma once|<code>#pragma once</code>]].

In [[Microsoft Visual C++]] (MSVC), there also exists an <code>#import</code> preprocessor directive, used to import type libraries.<ref>{{Cite web|url=https://learn.microsoft.com/en-us/cpp/preprocessor/hash-import-directive-cpp|title = #import directive (C++)}}</ref> It is a nonstandard directive.

The Objective-C directive should not be confused with the C++ keyword <code>import</code>, which is used to import C++ [[Precompiled header#Modules|modules]] (since [[C++20]]), and is not a preprocessor directive.

=== Nullable ===
The <code>#nullable</code> directive in C# is used to enable and disable nullable reference types. To enable them, use <code>#nullable enable</code>, and <code>#nullable disable</code> to disable them. 

<syntaxhighlight lang="csharp">
#nullable enable

string? name = null; // OK
string fullName = null; // Warning: possible null assignment

#nullable disable

string test = null; // No warning
</syntaxhighlight>

This directive does not exist in C/C++.

=== Region ===
The <code>#region</code> and <code>#endregion</code> directives in C# are used to expand/collapse sections of code in IDEs, and has no effect on actual compilation of the program. It is primarily used for code organisation and readability.
<syntaxhighlight lang="csharp">
#region Helper Methods

void Log(string message)
{
    Console.WriteLine(message);
}

#endregion
</syntaxhighlight>

This directive does not exist in C/C++.