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An inline function definition must be visible for the compiler to be able to inline a function at the point of the call. Unlike a non-inline function, an inline function must be defined in every text file in which the inline function is called. Of course, the definitions of the inline function that appear in the different files that compose a program must be the same. For a program made of two files compute.C and draw.C, a programmer must not define the inline function min() to mean one thing in compute.C and something else in draw.C. If the two definitions are not the same, the program has undefined behavior: it is uncertain which of the many different definitions the compiler will use as the definition to call for non-inlined function calls, and the program may not behave as you expect.

The recommended way to ensure that this does not happen is to place the definition of the inline function in a header file and to include this header file in every text file in which the inline function is called. This approach guarantees that there is only one definition for the inline function and that programmers do not unnecessarily duplicate code, something that may lead to unintentional mismatch later on in the lifetime of the program.