Why do we need to put private members in headers?

Question

Private variables are a way to hide complexity and implementation details to the user of a class. This is a rather nice feature. But I do not understand why in c++ we need to put them in the header of a class. I see two annoying downsides to this:

• It clutters the header from the user
• It force recompilation of all client libraries whenever the internals are modified

Is there a conceptual reason behind this requirement? Is it only to ease the work off the compiler?

Answer 1

It is because the C++ compiler must know the actual size of the class in order to allocate the right amount of memory at instantiation. And the size includes all members, also private ones.

One way to avoid this is using the Pimpl idiom, explained by Herb Sutter in his Guru of the Week series #24 and #28.

Update

Indeed, this (or more generally, the header / source file distinction and #includes) is a major hurdle in C++, inherited from C. Back in the days C++ C was created, there was no experience with large scale software development yet, where this starts to cause real problems. The lessons learned since then were heeded by designers of newer languages, but C++ is bound by backward compatibility requirements, making it really hard to address such a fundamental issue in the language.

Answer 2

The class definition needs to be sufficient for the compiler to produce an identical layout in memory wherever you've used an object of the class. For example, given something like:

class X { 
    int a;
public:
    int b;
};

The compiler will typically have a at offset 0, and b at offset 4. If the compiler saw this as just:

class X { 
public:
    int b;
};

It would "think" that b should be at offset 0 instead of offset 4. When code using that definition assigned to b, code using the first definition would see a get modified, and vice versa.

The usual way to minimize the effects of making changes to the private parts of the class is usually called the pimpl idiom (about which I'm sure Google can give a great deal of information).

Answer 3

There are most likely several reasons. While private members can't be accessed by most other classes, they can still be accessed by friend classes. So at least in this case they may be needed in the header, so the friend class can see they exist.

The recompilation of dependent files may depend on your include structure. Including the .h files in a .cpp file instead of another header can in some cases prevent long chains of recompilations.