Question

In the following C++ code:

struct Features {
  int F1;
  int F2;
  int F3;
  int F4;
  Features(int F1,int F2,int F3,int F4)
  : F1(F1), F2(F2), F3(F3), F4(F4) { }
};

What does this part mean?

Features(int F1,int F2,int F3,int F4)
      : F1(F1), F2(F2), F3(F3), F4(F4) { }

Thanks.

Answer 1

It is initializing the member variables using the constructor’s initializer list. it would be clearer if the names of the constructor parameters weren’t the same as the data members:

Features(int a,int b,int c,int d)
      : F1(a), F2(b), F3(c), F4(d) { }

It is useful to have some naming convention for data members, such that these are easily identifiable and distinguishable from local variables in code. Examples are prefixing an m_ or using a trailing _:

struct Features {
  int m_f1;
  int m_f2;
  int m_3f;
  int m_f4;
  Features(int f1,int f2,int f3,int f4)
  : m_f1(f1), m_f2(f2), m_f3(f3), m_f4(f4) { }
};

Both these constructors can be used like this:

Features f(11,22,33,44);
std::cout << f.m_f1 << "\n"; // prints 11
std::cout << f.m_f2 << "\n"; // prints 22
std::cout << f.m_f3 << "\n"; // prints 33
std::cout << f.m_f4 << "\n"; // prints 44

Note that the fact that this constructor has been defined means that the compiler will no longer provide a default constructor. So if you want to be able to to this:

Features f;

then you need to provide your own default constructor:

Features() : m_f1(), m_f2(), m_f3(), m_f4() {} // initializes data members to 0