C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 9:
[9.6] How do you tell the compiler to make a non-member function inline?

When you declare an inline function, it looks just like a normal function:

void f(int i, char c);
But when you define an inline function, you prepend the function's definition with the keyword inline, and you put the definition into a header file:
void f(int i, char c)
Note: It's imperative that the function's definition (the part between the {...}) be placed in a header file, unless the function is used only in a single .cpp file. In particular, if you put the inline function's definition into a .cpp file and you call it from some other .cpp file, you'll get an "unresolved external" error from the linker.