Define overloaded operators in the namespace of their operands

Reason

Readability. Ability for find operators using ADL. Avoiding inconsistent definition in different namespaces

Example

struct S { };
S operator+(S, S);   // OK: in the same namespace as S, and even next to S
S s;

S r = s + s;

Example

namespace N {
    struct S { };
    S operator+(S, S);   // OK: in the same namespace as S, and even next to S
}

N::S s;

S r = s + s;  // finds N::operator+() by ADL

Example, bad

struct S { };
S s;

namespace N {
    bool operator!(S a) { return true; }
    bool not_s = !s;
}

namespace M {
    bool operator!(S a) { return false; }
    bool not_s = !s;
}

Here, the meaning of !s differs in N and M. This can be most confusing. Remove the definition of namespace M and the confusion is replaced by an opportunity to make the mistake.

Note

If a binary operator is defined for two types that are defined in different namespaces, you cannot follow this rule. For example:

Vec::Vector operator*(const Vec::Vector&, const Mat::Matrix&);

This might be something best avoided.

See also

This is a special case of the rule that helper functions should be defined in the same namespace as their class.

Enforcement

• Flag operator definitions that are not in the namespace of their operands