Use `class` if the class has an invariant; use `struct` if the data members can vary independently

Reason

Readability. Ease of comprehension. The use of class alerts the programmer to the need for an invariant. This is a useful convention.

Note

An invariant is a logical condition for the members of an object that a constructor must establish for the public member functions to assume. After the invariant is established (typically by a constructor) every member function can be called for the object. An invariant can be stated informally (e.g., in a comment) or more formally using Expects.

If all data members can vary independently of each other, no invariant is possible.

Example

struct Pair {  // the members can vary independently
    string name;
    int volume;
};

but:

class Date {
public:
    // validate that {yy, mm, dd} is a valid date and initialize
    Date(int yy, Month mm, char dd);
    // ...
private:
    int y;
    Month m;
    char d;    // day
};

Note

If a class has any private data, a user cannot completely initialize an object without the use of a constructor. Hence, the class definer will provide a constructor and must specify its meaning. This effectively means the definer needs to define an invariant.

See also:

• define a class with private data as class
• Prefer to place the interface first in a class
• minimize exposure of members
• Avoid protected data

Enforcement

Look for structs with all data private and classes with public members.