What is stdafx.h and How Do Precompiled Headers Improve Compile Times in Visual Studio?

Understanding stdafx.h and Precompiled Headers

When working with Visual Studio IDE, you may encounter a file named "stdafx.h". This file plays a significant role in optimizing compile times. Let's delve deeper into its purpose and how precompiled headers work.

Purpose of stdafx.h

stdafx.h is an auto-generated file by Microsoft Visual Studio wizards. Its primary purpose is to define standard system and project-specific include files that are frequently used. These include files, such as those for common headers (e.g., stdio.h), provide foundational functionality for your application.

Precompiled Headers

stdafx.h leverages a technique called precompiled headers to enhance compilation efficiency. Compatible compilers, like Visual C 6.0 and later, utilize precompiled headers to reduce overall compile time by:

• Pre-compiling the contents of stdafx.h
• Assuming all code before the #include "stdafx.h" line in a source file is already compiled

This approach eliminates the need to recompile commonly used code each time you compile your project.

Compilation Behavior

Visual C defaults to compiling all code after the #include "stdafx.h" line. However, you can uncheck the /Yu'stdafx.h' compile option to disable the assumption that code prior to this line is precompiled.

Additional Considerations

• dafx.h: stdafx.h is platform-dependent. There is a separate dafx.h file for x64 platforms.
• Maintaining stdafx.h: It's important to keep stdafx.h up to date with necessary includes. If you add a new include file to your project, ensure you add it to stdafx.h as well.

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