C++ Core Guidelines: Class Rules

A class is a user-defined type where the programmer can specify the representation, the operations, and the interface. The C++ core guidelines have a lot of rules for user-defined types.

The guidelines start with quite general rules but also include special rules for constructors and destructors, class hierarchies, overloading of operators, and unions.

Before I write about the special rules that are way more interesting, here are the eight general rules.

• C.1: Organize related data into structures (structs or classes)
• C.2: Use class if the class has an invariant; use struct if the data members can vary independently
• C.3: Represent the distinction between an interface and an implementation using a class
• C.4: Make a function a member-only if it needs direct access to the representation of a class
• C.5: Place helper functions in the same namespace as the class they support
• C.7: Don’t define a class or enum and declare a variable of its type in the same statement
• C.8: Use class rather than struct if any member is non-public
• C.9: Minimize exposure of members

I will only write as much to the general class rules to make their intention clear.

General rules for classes

C.1: Organize related data into structures (structs or classes)

If data is related, you should put it into a struct or class; therefore, the second function is very easy to comprehend.

void draw(int x, int y, int x2, int y2);  // BAD: unnecessary implicit relationships
void draw(Point from, Point to);          // better

C.2: Use class if the class has an invariant; use struct if the data members can vary independently

An invariant is a logical condition that is typically established by a constructor.

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

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
};

The class Date has the invariants y, m, and d. They are initialized and checked in the constructor. The data type Pair has no invariant; therefore it is a struct.

Due to the invariant, the class is easier to use. This is exactly the aim of the next rule.

C.3: Represent the distinction between an interface and an implementation using a class

The public methods are in this case the interface of a class and the private part is the implementation.

class Date {
    // ... some representation ...
public:
    Date();
    // validate that {yy, mm, dd} is a valid date and initialize
    Date(int yy, Month mm, char dd);

    int day() const;
    Month month() const;
    // ...
};

From a maintainability perspective, the implementations of the class Date can be changed without affecting the user.

C.4: Make a function a member-only if it needs direct access to the representation of a class

If a function needs no access to the internals of the class, it should not be a member. Hence you get loose coupling and a change of the internals of the class will not affect the function.

C.5: Place helper functions in the same namespace as the class they support

Such a helper function should be in the namespace of the class.

namespace Chrono { // here we keep time-related services

    class Date { /* ... */ };

    // helper functions:
    bool operator==(Date, Date);
    Date next_weekday(Date);
    // ...
}
...
if (date1 == date2){  ...           // (1)

Thanks to argument-dependent lookup (ADL) the comparison in (1) will additionally look for the identity operator in the Chrono namespace.

C.7: Don’t define a class or enum and declare a variable of its type in the same statement

I admit: defining a class and declaring a variable of its type in the same statement confuses me.

// bad
struct Data { /*...*/ } data{ /*...*/ }; 

// good
struct Data { /*...*/ };         
Data data{ /*...*/ };

C.8: Use class rather than struct if any member is non-public

This is a quite useful and often used convention. If a data type has private or protected members, make it a class.

C.9: Minimize exposure of members

This rule is also called data hiding and is one of the cornerstones of object-oriented class design. It means that you should think about two interfaces for your class. A public interface for the general use case and a protected interface for derived classes. The remaining members should be private.

I will continue with the more special rules. Here is an overview:

• C.concrete: Concrete types
• C.ctor: Constructors, assignments, and destructors
• C.con: Containers and other resource handles
• C.lambdas: Function objects and lambdas
• C.hier: Class hierarchies (OOP)
• C.over: Overloading and overloaded operators
• C.union: Unions

Let's continue with the two rules to concrete types.

Concrete types

• C.10: Prefer concrete types over class hierarchies
• C.11: Make concrete types regular

First of all, I have to write about concrete types and regular types.

A concrete type is "the simplest kind of a class". It is often called a value type and is not part of a type hierarchy. Of course, an abstract type can not be concrete.

A regular type is a type that "behaves like an int" and has, therefore, to support copy and assignment, equality, and order. To be more formal. A regular type Regular supports the following operations.

•  Copy and assignment

Regular a;
Regular a  = b;
~Regular(a);
a = b;

• Equality

a == b;
a != b;

• Ordering

a < b;

The built-in types are regular such as the container of the standard template library.

C.10: Prefer concrete types over class hierarchies

If you have no use-case for a class hierarchy, use a concrete type. A concrete type is way easier to implement, smaller and faster. You have not to think about inheritance, virtuality, references or pointers including memory allocation and deallocation. There will be no virtual dispatch and, therefore, no runtime overhead.

You just have value.

C.11: Make concrete types regular

Regular types (ints) are easier to understand. They are per se intuitive. This means if you have a concrete type think about upgrading it to a regular type.

What's next

The next post will be about the lifetime of objects: create, copy, move, and destruct.