Require a complete set of operations for a concept

Reason

Ease of comprehension. Improved interoperability. Helps implementers and maintainers.

Note

This is a specific variant of the general rule that a concept must make semantic sense.

Example, bad

template<typename T> concept Subtractable = requires(T a, T b) { a - b; };

This makes no semantic sense. You need at least + to make - meaningful and useful.

Examples of complete sets are

• Arithmetic: +, -, *, /, +=, -=, *=, /=
• Comparable: <, >, <=, >=, ==, !=

Note

This rule applies whether we use direct language support for concepts or not. It is a general design rule that even applies to non-templates:

class Minimal {
    // ...
};

bool operator==(const Minimal&, const Minimal&);
bool operator<(const Minimal&, const Minimal&);

Minimal operator+(const Minimal&, const Minimal&);
// no other operators

void f(Minimal x, Minimal y)
{
    if (!(x == y)) { /* ... */ }    // OK
    if (x != y) { /* ... */ }       // surprise! error

    while (!(x < y)) { /* ... */ }  // OK
    while (x >= y) { /* ... */ }    // surprise! error

    x = x + y;          // OK
    x += y;             // surprise! error
}

This is minimal, but surprising and constraining for users. It could even be less efficient.

The rule supports the view that a concept should reflect a (mathematically) coherent set of operations.

Example

class Convenient {
    // ...
};

bool operator==(const Convenient&, const Convenient&);
bool operator<(const Convenient&, const Convenient&);
// ... and the other comparison operators ...

Convenient operator+(const Convenient&, const Convenient&);
// ... and the other arithmetic operators ...

void f(const Convenient& x, const Convenient& y)
{
    if (!(x == y)) { /* ... */ }    // OK
    if (x != y) { /* ... */ }       // OK

    while (!(x < y)) { /* ... */ }  // OK
    while (x >= y) { /* ... */ }    // OK

    x = x + y;     // OK
    x += y;        // OK
}

It can be a nuisance to define all operators, but not hard. Ideally, that rule should be language supported by giving you comparison operators by default.

Enforcement

Yes, std::string is "odd", but it's too late to change that.

• Flag classes that support "odd" subsets of a set of operators, e.g., == but not != or + but not -.