Constructors can initialise through assignment operator or initialisation lists. Benefit of using initialisation lists is the improved performance. Your code will run faster if you use initialization lists rather than assignment.

http://www.parashift.com/c++-faq-lit....html#faq-10.6

Another small thing to add that I read in the book C++ in Action .
Say you have a const member declared inside the C++ class.

C++ Syntax (Toggle Plain Text)

class Planet
{
public:
         Planet( int iWeight);
private:
         const int weight;
}

class Planet { public: Planet( int iWeight); private: const int weight; }

since the weight member cant be assigned a value in a standard contructor like this

C++ Syntax (Toggle Plain Text)

Planet::Planet( int iWeight)
{
       weight = iWeight ; // wrong
}

Planet::Planet( int iWeight) { weight = iWeight ; // wrong }

The only way you can do that is using an initialization list.

C++ Syntax (Toggle Plain Text)

Planet::Planet( int iWeight):weight(iWeight) // correct
{
}

Planet::Planet( int iWeight):weight(iWeight) // correct { }