构造函数初始化列表 - 这会有效吗？

Question

With your help I could solve the problem I described in this minimal example ( use parameterized constructor in other classes constructor ) using a constructor initializer list.

In my real life problem, things are a bit more difficult. My constructor now looks somehow like this:

ClassC::ClassC() : _objectB(_objectA.getInt()) {
  _objectA = ClassA();
}

Will this work? The compiler doesn't return any errors. However, _objectB is initialized with a value of _objectA , which is initialized in the line below. So how would _objectA.getInt() return a valid value?

If it doesn't work, how can I make it work?

Answer 1

If _objectA is declared before _objectB, then _objectA's default constructor will be called. And, _objectB will be initialzed with the getInt() value from _objectA. Afterward, in the body of the constructor, _objectA is set via assignment operator with the default constructor of ClassA--which is unneeded.

If _objectB is declared before _objectA, then IDK...it may be compiler implementation dependent.

Either way, you are basically initializing _objectA twice.

Answer 2

If _objectA is declared like

class ClassC
{
    ClassA _objectA;
    ClassB _objectB;

    public:
    ClassC();
}

it will work, because _objectA is already initialized (with the default constructor), and you just reassign it a new class later, which has nothing to do with _objectB initialization.

---

However, as @πάντα ῥεῖ suggested it is better to use

ClassC::ClassC() : _objectA(), _objectB(_objectA.getInt()) {}

that will initialize _objectA , then use it for _objectB initialization.

Answer 3

So how would _objectA.getInt() return a valid value?

Most probably yes. If objectA is initialized using the default constructor to yield valid values, it is already constructed implicitly (if the declaration appears before objectB ).

The assignment in the constructor body is superfluous.

Also it's better to write:

 ClassC::ClassC() : _objectA(), _objectB(_objectA.getInt()) {}

to make that clear in the member initializer list.

Answer 4

Class members are always initialized in the order they're declared, so assuming ClassC looks something like

class ClassC
{
    ClassA _objectA;
    ClassB _objectB;

public:
    ClassC();
//...
};

then your constructor is equivalent to

ClassC::ClassC() : _objectA(), _objectB(_objectA.getInt()) {
    _objectA = ClassA();
}

As you can see, _objectA is initialized before _objectB and then later you copy-assign to _objectA from an anonymous ClassA object in the constructor's body.