Feature Check Macros

At compile time, we sometimes need to conditionally compile code based on whether a language feature is provided by the underlying platform or compiler for various purposes, such as portability or performance. In such cases, we define feature check macros in absl/config.h to check whether the feature is available or missing. Introducing a feature check macro is preferable to writing a complex conditional directly because it explicitly states which feature it is checking and makes the code more readable and maintainable.

Two types of feature check macros exist:

  • Compiler checks
  • Platform checks

In Abseil, feature check macros are named with an ABSL_* prefix and defined to 1 for a “true” state, and undefined for a “false” state. Defining them in this manner allows you to use either #if or #ifdef constructs interchangeably.

Example:

#ifdef ABSL_HAVE_FEATURE_FOO
#error "ABSL_HAVE_FEATURE_FOO cannot be overridden."
#elif <complex preprocessor conditional>
#define ABSL_HAVE_FEATURE_FOO 1
#endif

Using Feature Check Macros

Feature check macros are used to conditionally compile code.

Example:

#if ABSL_HAVE_FEATURE_FOO  // or #ifdef ABSL_HAVE_FEATURE_FOO
// Handle the case where feature foo is available.
#else
// Handle the case where feature foo is missing.
#endif

Writing Your Own Feature Check Macros

Feature check macros are not allowed to be overridden (-D) on the compiler command line. They should only be derived from pre-defined macros, other feature check macros, or build configuration macros. For reusability, a feature check macro should only be added to absl/base/config.h if it is used in at least three distinct modules.

Note: #if and #ifdef have different results when a macro is defined to 0. We explicitly state that a feature check macro should be undefined for a “false” state so both #if and #ifdef can be used interchangeably.

When writing preprocessor conditionals, you can either opt into use of a feature by explicitly listing all platforms where a given feature is available, or opt out by explicitly listing all platforms where a given feature is missing or broken. Either option has pros and cons:

| | opt-in | opt-out |
| —- | ——————————- | ———————————– |
| pros | Generally safe, and more likely | The compilation fails if a new |
: to build on a new platform. : platform doesn’t support the :
: : feature. : | cons | The allow-list needs to be | Compilation might succeed but |
: extended for each new platform. : runtime behavior might be :
: : unexpected, if the interface exists :
: : but the implementation is :
: : problematic. :

When choosing a opt-in or opt-out approach, think about what is likely to happen when a new platform is added and which approach is easier to maintain the invariant of your module.

Some examples:

If you are working around a compiler bug or a missing C++ feature in the standard library implementation, you probably want to deny-list the combinations where the feature is missing. When a new platform is added, you can assume it is standards-compliant, and your unit test will fail if it is not.

If you are relying on a low-level OS feature for better functionality (more logging, better debugging context), you probably want to allow-list the operating systems that claim to support this feature. When a new OS is added, you can assume the feature is missing, and your module will continue to work with minimum functionality. If desired, a developer can decide to opt-in by extending the allow-list.