Use `class` rather than `struct` if any member is non-public

Reason

Readability. To make it clear that something is being hidden/abstracted. This is a useful convention.

Example, bad

struct Date {
    int d, m;

    Date(int i, Month m);
    // ... lots of functions ...
private:
    int y;  // year
};

There is nothing wrong with this code as far as the C++ language rules are concerned, but nearly everything is wrong from a design perspective. The private data is hidden far from the public data. The data is split in different parts of the class declaration. Different parts of the data have different access. All of this decreases readability and complicates maintenance.

Note

Prefer to place the interface first in a class, see NL.16.

Enforcement

Flag classes declared with struct if there is a private or protected member.