FAQ 18.06 Why is private: the default access level for a class?

FAQ 18.06 Why is private: the default access level for a class?

The default assumption makes small programs easier to read. For example, when a member function is defined within the class body proper (a practice that we are not advocating), it is easier to understand the code if the data appears before the code. Note that the compiler finds the class's data member independent of whether it appears before or after the usage, but human readers don't usually do a two-pass scan. Since the data is often private:, the default access level makes it a tiny bit easier when functions are defined within the class body proper. For example, this is particularly valuable for textbook examples that define member functions within their class to save presentation space.

But typical development efforts separate a lot of the implementation into a distinct file, such as a .cpp file. Thus, the problem with the member function coming before the data doesn't exist, and there is more reason to put the public: portion first. This lets the human reader see the interface portion of the class without having to wade through irrelevant implementation detail.

Different authors take different approaches to this issue, but we probably all subscribe to the same guiding principle. Any differences are due to our audiences and presentation style rather than philosophies.