Code in header files will always be inlined?

Question

I just had a discussion with a coworker concerning code in header files:

He says that code defined in header files will always be inlined by the compiler (like the code from the function GetNumber() in my example header). I say it will be inlined sometimes, whenever the compiler decides to do so. So which one of us has to bring a cake to work for telling filthy lies? Or maybe we are both wrong...?

MyClass.hpp

   class MyClass
    {
    public:
    MyClass();
    ~MyClass();

    int GetNumber() const 
    {
     //...; 
     return m_number;
    };

    private:
    int m_number;
    };

Answer 1

Any function defined within the class (like your GetNumber example) rather than just declared is implicitly inline. What that means is that it's equivilent to using the inline keyword, so multiple inclusions of the header will not cause link errors due to multiple definitions of those functions.

Most modern compiler treat inline as a linkage command and nothing more. Some compilers provide stronger keywords such as CL's __forceinline which mean 'inline this if it's possible to do so'.

So you're both right and both wrong to a degree.

Answer 2

Your pal is wrong, you're right.

Inlining do not depends on where the code is (header or not). After preprocessing there is no headers or non-headers. Whole unit is a single file, it contains all included stuff.

Try running gcc preprocessor then you will see:

gcc -E some_source_file_with_includes