The inline keyword tells the compiler to substitute the code within the function definition for every instance of a function call. Using inline functions can make your program faster because they eliminate the overhead associated with function calls.

An inline function is one for which the compiler copies the code from the function definition directly into the code of the calling function rather than creating a separate set of instructions in memory.

1. Inlining as an Optimization: The primary purpose of defining a function as “inline” is to suggest to the compiler that, instead of making a standard function call (which includes pushing arguments onto the call stack, executing the function, and returning), it should insert the actual code of the function directly at the call site.
2. Function Size: Inline functions are typically small and simple functions because inlining large and complex functions can lead to larger compiled code and potential performance issues.
3. Benefits: Inlining can potentially lead to faster code execution because it eliminates the overhead of function call and return operations. It can be particularly beneficial for small, frequently used functions, like accessors or mutators in object-oriented programming.
4. Trade-offs: Inlining comes with trade-offs. While it can improve performance, it can also increase code size, potentially leading to larger executables. Additionally, inlining functions that are not frequently used can be counterproductive, as it can bloat the code unnecessarily.
5. Compiler Discretion: The decision to inline a function is ultimately at the discretion of the compiler. The “inline” keyword serves as a suggestion rather than a strict command. The compiler will consider various factors, such as function size and how often it is called, before deciding whether to inline a function.