Memberwise Initialization [duplicate]

Question

Possible Duplicate:
C++ initialization lists

What is the difference between member-wise initialization and direct initialization in a class? What is the difference between the two constructors defined in the class?

class A
{
    public:
    int x;
    int y;
    A(int a, int b) : x(a), y(b)
    {}

    A(int a, int b)
    {
        x = a;
        y = b;
    }
};

Answer 1

The theoretical answers have been given by other members.

Pragmatically, member-wise initialization is used in these cases :

• You have a reference attribute (MyClass & mMyClass) in your class. You need to do a member-wise initialization, otherwise, it doesn't compile.
• You have a constant attribute in you class (const MyClass mMyClass). You also need to do a member-wise initialization, otherwise, it doesn't compile.
• You have an attribute with no default constructor in your class (MyClass mMyClass with no constructor MyClass::MyClass()). You also need to do a member-wise initialization, otherwise, it doesn't compile.
• You have a monstruously large attribute object (MyClass mMyClass and sizeof(MyClass) = 1000000000). With member-wise initialization, you build it only once. With direct initialization in the constructor, it is built twice.

Answer 2

First one uses initialization and second one does NOT use initialization, it uses assignment. In the second one, the members x and y are default-initialized first (with zero), and then they're assigned with a and b respectively.

Also note that in the second one, members are default initialized only if the types of members are having non-trivial default constructor. Otherwise, there is no initialization (as @James noted in the comment).

Now see this topic to know:

• What is a non-trivial constructor in C++?