用C ++调用构造函数

Question

Memory is allocated to variables inside a class only after the objects are created ..right? So what if the class contain a variable which is an object of another class and the variable has a constructor?

class Wand
{
    mouse mouseEmu(0,0);
    QCursor pt;
};

mouseEmu and QCursor are two classes...

When will be the constructor of mouseEmu called and when will be the default constructor for pt called?

Is it necessary to call the parametrised constructor if we specify one?

Answer 1

when will be the constructor of mouseEmu called and when will be the default constructor for pt called?

All data member and base-class sub-objects of the currently constructed object all fully constructed before the body of the constructor of the current object is entered. So if your class had a user-defined constructor, then by the time the code inside the user-defined constructor is executed, all the other sub-objects of the class have had their constructors completed.

If for some reason you need to setup a user-defined initialized value for a sub-object, and that sub-object has a non-default constructor, then you can use an initialization list with the constructor of the current object to pass values to the constructor of the sub-object. For instance:

class Wand
{
    mouse mouseEmu;
    QCursor pt;

    Wand(): mouseEmu(0,0) {} //empty constructor body with initialization list
};

Answer 2

New Question (or old depending on when you arrived)

class Wand
{
    mouse mouseEmu(0,0);
             //   ^^^^^^  This bit is illegal.
             //           Remove it here. You specify the parameters
             //           Passed to members in the constructor.
    QCursor pt;
};

What you want is:

class Wand
{
    Ward(): mouseEmu(0,0) {}
    mouse mouseEmu;
    QCursor pt;
};

Edit:

Answer based on the question before it was changed to illegal.

If you don't provide a constructor then the compiler plants an implicit default constructor for you:

Wand::Wand()
  : mouseEmu()
  , pt()
{}

Thous when you create an object of Wand. It will automatically create and initialize its members (mouseEmu and pt) as part of the construction of the Wand object. Calling their constructors (in the order declared in the class).

Note: If you define your own constructor. But don't explicitly call the constructor of your members then the members default constructor are implicitly called (the order of construction of members is ALWAYS the order of declaration.

 // Example:
 // If you did:

 class Wand
 {
      Wand(int x) : mouseEmu(0,x) {}

      // The above will generate this logical code.
      Wand(int x) : mouseEmu(0,x), pt() {}
 }

Answer 3

That's not legal C++ syntax. It looks like you're declaring a function, but 0 is not a valid type identifier.

Constructors of subobjects (bases and member variables) are called from the constructors of your class, including compiler-generated constructors.

The ctor-initializer-list allows you to specify parameters to subobject constructors. It looks like this:

class Wand
{
    mouse mouseEmu;

    QCursor pt;
public:
    Wand() : mouseEmu(0,0) {}
};

This way, your mouseEmu subobject is initialized with a two-parameter constructor, and pt is initialized by its default (zero-parameter) constructor.

The compiler-generated copy-constructor will initialize both subobjects by calling copy-constructors and passing the members of the Wand object being copied.