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

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

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