Make copy assignment safe for self-assignment

Reason

If x = x changes the value of x, people will be surprised and bad errors will occur (often including leaks).

Example

The standard-library containers handle self-assignment elegantly and efficiently:

std::vector<int> v = {3, 1, 4, 1, 5, 9};
v = v;
// the value of v is still {3, 1, 4, 1, 5, 9}

Note

The default assignment generated from members that handle self-assignment correctly handles self-assignment.

struct Bar {
    vector<pair<int, int>> v;
    map<string, int> m;
    string s;
};

Bar b;
// ...
b = b;   // correct and efficient

Note

You can handle self-assignment by explicitly testing for self-assignment, but often it is faster and more elegant to cope without such a test (e.g., using swap).

class Foo {
    string s;
    int i;
public:
    Foo& operator=(const Foo& a);
    // ...
};

Foo& Foo::operator=(const Foo& a)   // OK, but there is a cost
{
    if (this == &a) return *this;
    s = a.s;
    i = a.i;
    return *this;
}

This is obviously safe and apparently efficient. However, what if we do one self-assignment per million assignments? That's about a million redundant tests (but since the answer is essentially always the same, the computer's branch predictor will guess right essentially every time). Consider:

Foo& Foo::operator=(const Foo& a)   // simpler, and probably much better
{
    s = a.s;
    i = a.i;
    return *this;
}

std::string is safe for self-assignment and so are int. All the cost is carried by the (rare) case of self-assignment.

Enforcement

(Simple) Assignment operators should not contain the pattern if (this == &a) return *this; ???