Before C++11

In the era of C++98/03, each compiler introduced its own set of annotations, usually with a different keyword.

Often, you could see code with #pragma, __declspec, __attribute spread throughout the code.

Here’s the list of the common syntax from GCC/Clang and MSVC:

GCC Specific Attributes

GCC uses annotation in the form of __attribute__((attr_name)). For example:

int square(int) __attribute__ ((pure)); // pure function

Documentation:

• Attribute Syntax - Using the GNU Compiler Collection (GCC)
• Using the GNU Compiler Collection (GCC): Common Function Attributes

MSVC Specific Attributes

Microsoft mostly used __declspec keyword, as their syntax for various compiler extensions.

See the documentation here: __declspec Microsoft Docs.

__declspec (deprecated) void LegacyCode() {}

Clang Specific Attributes

Clang, as it’s straightforward to customize​, can support different types of annotations. Most of GCC attributes work with Clang.

See the documentation here: Attributes in Clang — Clang documentation.