When Should I Use Inline Functions in C ?

When to Use and Not Use Inline Functions

Inline functions offer potential performance gains by eliminating the overhead of function calls. However, their use should be judicious, considering the following criteria:

When to Use Inline Functions:

• Small functions: Small functions with limited code complexity are ideal candidates for inlining. Their integration into the calling context can significantly reduce execution time.
• Frequent calls: Functions that are called multiple times within a code block benefit from inlining for similar reasons.
• Avoidance of #define: When defining simple functions, prefer inline over #define to preserve type safety and scope.

When to Avoid Inlining:

• Large functions: Inlining large functions can result in large executables, which can negatively impact performance.
• I/O-bound functions: I/O operations involve significant overhead outside the scope of function calls and are not suited for inlining.
• Less frequently used functions: Functions that are called infrequently may not justify the potential code bloat resulting from inlining.
• Constructors and destructors: Inline constructors and destructors can lead to compiler-generated code, which may not be necessary.

Binary Compatibility Considerations:

• Avoid inlining existing functions: Inlining previously defined functions may break binary compatibility.
• Provide non-inline implementations: For library development, provide non-inline implementations of virtual destructors, constructors, and copy operators to ensure extensibility.

Other Considerations:

• Compiler discretion: Inline is a hint to the compiler, which may choose not to inline a function or inline non-marked ones.
• Profiling: Consider using profiling to identify bottleneck functions that could benefit from inlining.

Additional Insights from References:

• [To Inline or Not To Inline](https://www.devwars.com/blogs/jack-allen/a/to-inline-or-not-to-inline)
• [9] Inline functions](https://en.cppreference.com/w/cpp/language/inline)
• [Policies/Binary Compatibility Issues With C ](https://isocpp.org/wiki/faq/backward-compatibility#binary-compatibility)
• [GotW #33: Inline](https://gotw.ca/publications/mill16.htm)
• [Inline Redux](https://www.foonathan.net/2017/01/01/inline-redux/)
• [Effective C - Item 33: Use inlining judiciously](https://www.aristeia.com/Book/Book_Effective_CPP/33-use-inlining-judiciously/)

Bjarne Stroustrup's Perspective:

Struostroup emphasizes that inlining is a preference expressed to the compiler, which may or may not implement it. The definition of an inline function must be available in scope to enable inlining.

ISO-IEC Standard Definition:

The ISO-IEC 14882-1998 standard defines inline functions as declarations with an inline specifier. It indicates a preference for inline substitution but does not guarantee its execution. Inline functions maintain their unique addresses and static variables.

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