What is the member variables list after the colon in a constructor good for?

Question

I\'m reading this C++ open source code and I came to a constructor but I don\'t get it ( basically because I don\'t know C++ :P )

I understand C and Java very well.

Answer 1

Without using the initialiser list all class members will simply have their default constructor called so this is the only place that you can control which constructor is called (for non-dynamically allocated members). The same is true for which parent class constructor will be called.

Class members "initialised" within the body of the constructor (i.e. between the {} braces using the = operator) isn't technically initialisation, it's an assignment. For classes with a non-trivial constructor/destructor it can be costly to default construct and then modify through assignment in this way. For reference members you must use the initialiser list since they cannot be changed via the assignment operator.

If the member (or parent class) does not have a default constructor then failing to specify an appropriate constructor in the initialiser list will cause the compiler to generate an error. Otherwise the compiler will insert the default constructor calls itself. For built in types this does nothing so you will have garbage values there.

Note that the order in which you specify the members in the initialiser list does not affect the order in which they are called. It is always the parent class constructor (if any) first, then the class members in the order in which they are defined in the class definition. The order in which you put them in the initialiser list does not matter and can be the source of subtle bugs...

In the contrived example below it looks like the intention is to initialise m_b with value then m_a with m_b, but what actually happens is that m_a is initialised with m_b (which is itself not yet initialised) then m_b gets initialised with value. m_b will just contain garbage!

struct BadInitialiserListExample
{
    BadInitialiserListExample(int value) :
        m_b(value),
        m_a(m_b)      // <-- *BUG* this is actually executed first due to ordering below!
    {
    }

    int    m_a;
    int    m_b;
};