Use an operator for an operation with its conventional meaning

Reason

Readability. Convention. Reusability. Support for generic code

Example

void cout_my_class(const My_class& c) // confusing, not conventional, not generic
{
    std::cout << /* class members here */;
}

std::ostream& operator<<(std::ostream& os, const my_class& c) // OK
{
    return os << /* class members here */;
}

By itself, cout_my_class would be OK, but it is not usable/composable with code that relies on the << convention for output:

My_class var { /* ... */ };
// ...
cout << "var = " << var << '\n';

Note

There are strong and vigorous conventions for the meaning of most operators, such as

• comparisons (==, !=, <, <=, >, >=, and <=>),
• arithmetic operations (+, -, *, /, and %)
• access operations (., ->, unary *, and [])
• assignment (=)

Don't define those unconventionally and don't invent your own names for them.

Enforcement

Tricky. Requires semantic insight.