C++: Where to initialize variables in constructor

Question

Possible Duplicate:
C++ initialization lists

What are the pros/cons of initializing variables at option 1 vs option 2?

class MyClass
{
public:
    MyClass( float f, char a );
private:
    float mFloat;
    char mCharacter;
    bool mBoolean;
    int mInteger;
};

MyClass::MyClass( float f, char a ) : mFloat( f ), mBoolean( true ) // option 1.
{
    // option 2
    mCharacter = a;
    mInteger = 0;
}

Edit: Why is option 2 so common?

Answer 1

In short, always prefer initialization lists when possible. 2 reasons:

• If you do not mention a variable in a class's initialization list, the constructor will default initialize it before entering the body of the constructor you've written. This means that option 2 will lead to each variable being written to twice, once for the default initialization and once for the assignment in the constructor body.

• Also, as mentioned by mwigdahl and avada in other answers, const members and reference members can only be initialized in an initialization list.

Also note that variables are always initialized on the order they are declared in the class declaration, not in the order they are listed in an initialization list (with proper warnings enabled a compiler will warn you if a list is written out of order). Similarly, destructors will call member destructors in the opposite order, last to first in the class declaration, after the code in your class's destructor has executed.