Define and initialize data members in the order of member declaration

Reason

To minimize confusion and errors. That is the order in which the initialization happens (independent of the order of member initializers).

Example, bad

class Foo {
    int m1;
    int m2;
public:
    Foo(int x) :m2{x}, m1{++x} { }   // BAD: misleading initializer order
    // ...
};

Foo x(1); // surprise: x.m1 == x.m2 == 2

Enforcement

(Simple) A member initializer list should mention the members in the same order they are declared.

See also: Discussion