Why Must Inline Functions Seemingly Reside in Header Files?

Inline Function Placement

Question: Why do inline functions seemingly need to be defined in a header file?

Explanation:

The inline definition of a class function does not mandatorily reside in a header file. However, the One Definition Rule (ODR) mandates that inline functions have an identical definition in every translation unit that utilizes them.

Adhering to the ODR is most readily accomplished by including the definition in a header file, which ensures availability to all translation units.

If restricting function definition to a single source file is desired, avoid declaring it inline. This allows the compiler to inline the function upon necessity.

Deciding whether to declare a function inline primarily depends on the desired compliance with the ODR. Enforcing inline by adding the keyword restriction may not be optimal.

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