How do you declare a class inline?

Question

My homework assignment is telling me to declare a class inline, but I've only heard of declaring functions inline. What does an inline class declaration do, and how do you achieve it?

Answer 1

I think they are just saying that your classes should go in the header, and for convenience all the class methods should be declared inline in the class itself, instead of putting the class methods out-of-line or into a separate CPP.

It's slightly ambiguous but that's how I'd interpret it.

Answer 2

Comparing functions and classes:

A function can have a declaration and a definition:

ReturnType FunctionName(Parameter1, Parameter2); // a function declaration

ReturnType FunctionName(Parameter1, Parameter2) // a function definition
{
    // do something
    return ReturnType;
}

The function declaration gives all the information anyone needs who wants to call the function.

The function definition allows the compiler to generate machine code so that when someone calls the function there will be instructions somewhere in the executable that will be called.

This generated code exists only once in the executable. Usually you would put the function definition in a .cpp file (or .c if you're using C, this applies there too), which is compiled and linked into the executable. If that .cpp file isn't linked you will get a linker error saying FunctionName not found or something like that.

If you put the function definition in a header file it is more than likely that the header will be used in multiple places in your system, and included into other .cpp files. This will produce the opposite problem at link time FunctionName already defined in this, that, or the other object file.

So, functions usually aren't defined in header files because the header is usually included in multiple .cpp files so would end up producing duplicated object code.

With classes however it's slightly different. You have two different ways of defining a function for a class. One way is similar to how functions are defined above:

//myleanklass.h
class MyLeanKlass
{
public:
    void perform();
};

//myleanklass.cpp
void MyLeanKlass::perform()
{
    // Sing, dance, and play the piano
}

Just as with a free function we could have put the definition in the header file, as long as it wasn't included in more than one .cpp file. Otherwise we'd get a similar linker error again void MyLeanKlass::perform already defined ... This scenario is not the norm though. You usually declare classes so that they can be reused through a system. So the header will be included in multiple places and lead to linker errors.

Just to be clear, we could have put the definition in the header file, but not inline. You wouldn't be able to include this header multiple times in your system:

class MyLeanKlass
{
public:
    void perform();
};

void MyLeanKlass::perform()
{
    // Sing, dance, and play the piano
}

Finally then, we get to the answer to the question. You can define class functions in the header file as long as they are within the class declaration. This is called inline definition. Even if the header is included in multiple .cpp files it won't lead to multiple definitions in compilation units and won't cause linker errors.

//myleanklass.h
class MyLeanKlass
{
public:
    void perform()
    {
        // Sing, dance, and play the piano
    }
};

However, there are problems with this approach:

• Inlining class functions is likely to make compilation take longer as the compiler will still compile every repeated definition of class functions and only throw away the duplicates at link time.
• Making changes to class functions causes a recompilation of any .cpp file which includes it, again leading to longer re-compilation times.
• Sometimes it just won't be possible. If a class uses another class and that uses the other class, you're going to have a cyclic dependency and it won't be possible to define everything inline. You can solve that by just putting the problem functions into a separate .cpp file.