C++ defining a constant member variable inside class constructor

Question

Usually when you have a constant private member variable in your class, which only has a getter but no setter, it would look something like this:

// Example.h
cl         


        

Answer 1

You have two basic options. One is to use the conditional operator, which is fine for simple conditions like yours:

Example::Example(const std::vector &myVec)
  : m_value( myVec.size() ? myVec.size() + 1 : -1)
{}

For more complex things, you can delegate the computation to a member function. Be careful not to call virtual member functions inside it, as it will be called during construction. It's safest to make it static:

class Example
{
  Example(const std::vector &myVec)
    : m_value(initialValue(myVec))
  {}

  static int initialValue(const std::vector &myVec)
  {
    if (myVec.size()) {
      return myVec.size() + 1;
    } else {
      return -1;
    }
  }
};

The latter works with out-of-class definitions as well, of course. I've placed them in-class to conserve space & typing.