Qt Creator: “inline function used but never defined” – why?

Question

Why am I getting this warning in Qt Creator: ` inline function ‘bool Lion::growl ()’ used but never defined?

I double-checked my code, and have a declaration

inline bool growl () in Lion (lion.h)

and the corresponding implementation in lion.cpp:

inline bool Lion::growl ()

What’s going on?

EDIT: My assumption has been that it is legal to define the actual inline method in the .cpp file (the inline keyword alerts the compiler to look for the method body elsewhere), or am I mistaken?

I don't want to clutter my header files with implementation details.

Answer 1

Well, I don't know the exact problem, but for starters:

• Inline methods are supposed to be implemented in the header file. The compiler needs to know the code to actually inline it.
• Also using the "inline" keyword in the class declaration doesn't have any effect. But it cannot hurt either.

See also: c++ faq lite

Answer 2

Inline methods are supposed to be implemented in the header file. The compiler needs to know the code to actually inline it.

Except if the inline function is used in the same project, possibly in another file that #include its header.

I miss there is such a restriction for libraries because restricting headers to function prototypes make things more readable.

What about #include-ing the .cpp ?

Answer 3

In addition to what Johan said, you cannot have a separate definition and declaration for the function even if both are in the same header file. This holds true especially for member functions of classes. The function code should be of the form:

class someClass
{
void someFunc()
{ ... }
}
// This will make the function inline even w/o the explicit 'inline'

And NOT of the form

class someClass
{
public:
     void someFunc();
}

void someClass::someFunc()
{ ... }