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Why do classes in C++ have to declare their private functions? Has it actual technical reasons (what is its role at compile time) or is it simply for consistency's sake?
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Private methods are useful for breaking tasks up into smaller parts, or for preventing duplication of code which is needed often by other methods in a class, but should not be called outside of that class.
You should make a function private when you don't need other objects or classes to access the function, when you'll be invoking it from within the class. Stick to the principle of least privilege, only allow access to variables/functions that are absolutely necessary.
So what's the difference between a public and private function? A private function can only be used inside of it's parent function or module. A public function can be used inside or outside of it. Public functions can call private functions inside them, however, since they typically share the same scope.
Actually, it is not required that a function be declared before use in C. If it encounters an attempt to call a function, the compiler will assume a variable argument list and that the function returns int.
I asked why private functions had to be declared at all, as they don't add anything (neither object size nor vtable entry) for other translation units to know
If you think about it, this is similar to declaring some functions static in a file. It's not visible from the outside, but it is important for the compiler itself. The compiler wants to know the signature of the function before it can use it. That's why you declare functions in the first place. Remember that C++ compilers are one pass, which means everything has to be declared before it is used.1
From the programmer's point of view, declaring private functions is still not completely useless. Imagine 2 classes, one of which is the friend of the other. The friendzoned class2 would need to know how the privates of that class look like, (This discussion is getting weird) otherwise they can't use it.
As to why exactly C++ was designed in this way, I would first say there is the historical reason: the fact that you can't slice a struct in C, was adopted by C++ so you can't slice a class (and adopted by other languages branched from C++, too). I'd also guess that it's about simplicity: Imagine how difficult it would be to devise a method of compilation in which you can split the class among different header files, let your source files know about it, and prevent others from adding stuff to your class.
A final note is that, private functions can affect vtable size. That is, if they are virtual.
1Actually not entirely. If you have inline functions in the class, they can refer to functions later defined in the same class. But probably the idea started from single pass and this exception later added to it.
2It's inlined member functions in particular.
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