Question

There are two ways construct a class:

  class Cell{
  public:
        Cell(int cellID, int nx);
        ~Cell();
  private:
        int cellID_;
        int nx;
  };

The first way:

  Cell::Cell(int cellID, int nx)
    : cellID_(cellID), nx_(nx){}

The second way :

  Cell::Cell(int cellID, int nx){init(cellID, nx)}

  void Cell::init(int cellID, int nx){
         cellID_ = cellID;
         nx_ = nx;
  }

Answer 1

Performance:

The first one is the best because it initializes the objects in true sense unlike second method which assigns the already initialized objects.

Note that there is a little overhead when you use the second method:

As you see there is an additional overhead of creation & assignment in the latter, which might be considerable for user defined classes.

Cost of Member Initialization = Object Construction 
Cost of Member Assignment = Object Construction + Assignment

In case of members which are in-built/POD data types there is no overhead but if the members are non POD types then the overhead is significant.

Necessity:

Note that You will be forced to use the member initializer list in certain scenarios:

• Your class has a reference member
• Your class has a non static const member

Such members cannot be assigned to but they must be initialized in member initializer list.

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Given the above as a practice the first method is always preferrable.