Use `=default` if you have to be explicit about using the default semantics

Reason

The compiler is more likely to get the default semantics right and you cannot implement these functions better than the compiler.

Example

class Tracer {
    string message;
public:
    Tracer(const string& m) : message{m} { cerr << "entering " << message << '\n'; }
    ~Tracer() { cerr << "exiting " << message << '\n'; }

    Tracer(const Tracer&) = default;
    Tracer& operator=(const Tracer&) = default;
    Tracer(Tracer&&) noexcept = default;
    Tracer& operator=(Tracer&&) noexcept = default;
};

Because we defined the destructor, we must define the copy and move operations. The = default is the best and simplest way of doing that.

Example, bad

class Tracer2 {
    string message;
public:
    Tracer2(const string& m) : message{m} { cerr << "entering " << message << '\n'; }
    ~Tracer2() { cerr << "exiting " << message << '\n'; }

    Tracer2(const Tracer2& a) : message{a.message} {}
    Tracer2& operator=(const Tracer2& a) { message = a.message; return *this; }
    Tracer2(Tracer2&& a) noexcept :message{a.message} {}
    Tracer2& operator=(Tracer2&& a) noexcept { message = a.message; return *this; }
};

Writing out the bodies of the copy and move operations is verbose, tedious, and error-prone. A compiler does it better.

Enforcement

(Moderate) The body of a user-defined operation should not have the same semantics as the compiler-generated version, because that would be redundant.