_DEBUG vs. NDEBUG: When Should I Use Each Preprocessor Define for Debugging?

_DEBUG vs NDEBUG: Understanding Preprocessor Defines for Debug Sections

When it comes to specifying debug sections of code, two preprocessor defines often come into play: _DEBUG and NDEBUG. These defines are used to control the inclusion or exclusion of code based on the compilation settings.

_DEBUG

_DEBUG is a preprocessor define that is commonly used in Microsoft Visual Studio (MSVC). It is defined to 1 by default when building in debug mode (/MTd or /MDd options). By default, code that contains #ifdef _DEBUG or #if defined(_DEBUG) preprocessor directives will be compiled only when building in debug mode. This approach helps isolate debugging code from release builds to prevent any performance overheads.

NDEBUG

NDEBUG, on the other hand, is a preprocessor define that is standardized in C (since C99) and C . It is defined to 0 by default and is intended to disable the standard-C assertions (). Code that uses NDEBUG directives is typically excluded from release builds, as assertions can introduce runtime overhead.

Which Define to Use?

The choice of which define to use depends on the specific debugging needs of your code and your development environment.

• _DEBUG: If your code uses MSVC-specific debugging techniques, it is recommended to use _DEBUG.
• NDEBUG: If your code is primarily concerned with disabling standard-C assertions, NDEBUG is a suitable choice.

Alternatives to Preprocessor Defines

While preprocessor defines are a common approach to controlling debug sections, they may introduce naming collisions. To avoid this, it is also possible to define custom debugging macros to represent debug sections. However, it is important to avoid starting these names with an underscore, as they are reserved by the compiler or C runtime.

Example Usage

To use _DEBUG or NDEBUG effectively, you would include directives like the following in appropriate sections of your code:

#ifdef _DEBUG
  // Code to only be compiled in debug mode
#endif

#ifndef NDEBUG
  // Code to only be compiled in release mode
#endif

By understanding the implications of using _DEBUG and NDEBUG, you can effectively control the compilation of debug sections in your code, ensuring that your code behaves as intended in both debug and release environments.

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