inline function limitations

inline function limitations

The inline specifier is wonderful when it works. A very important point to note about inline functions is that the inline specifier, by itself, is not enough to guarantee that inline code will be generated. The other requirement is that the compiler must know the function body code, where the function is called.

Hence, an inline keyword in a function prototype declaration is not enough. The executable statements inside the function’s definition (i.e., the function body) must be available to the C++ compiler. Otherwise, how is the compiler to know what inline code to expand a function call into? I guess in theory the C++ compiler could maintain a huge database of all the functions in your source code, or scan through all the CPP files to find it, and that would be amazing, but we're not there yet. In practice, the compiler will only inline functions where it has seen the function body within the current C++ source file or an included header file. This requirement imposes two restrictions on the use of inline functions:

1. Member functions declared as inline should include the function body inside the same header file as the class declaration. This can be achieved by placing the function body of a member function inside the class declaration. For a more readable style when there are many inline member functions, the class declaration can declare the function prototypes, and then provide the inline function definitions immediately after it, in the same header file. This restriction ensures that whenever the class declaration is included as a header file, the member function body is available for inlining.

2. Non-member inline functions must be defined before they are used within a source file, preferably by placing the inline functions in a header file. Placing inline functions at the top of a source file allows the inlining of any function calls later in the same source file, but calls to the functions from a different source file cannot be inlined by the compiler unless the inline function definition is placed in a header file.

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