C ++ What to add to the class declaration and what not

Question

I am completely confused where to put, for example. constructor definition. Sometimes you see something like this

// point.h
class Point {
  Point(int x, int y) {
    x_ = x;
    y_ = y;
  }
private:
  const int x_;
  const int y_;
}

Then you sometimes see something like this:

// point.h
class Point {
  Point(int x, int y);
private:
  const int x_;
  const int y_;
}

// point.cc
Point::Point(int x, int y) {
  x_ = x;
  y_ = y;
}

those. sometimes things like constructors, copy constructors, etc., are declared in .h, and then implemented in a file .cc, sometimes they are defined in the header, etc. But under what circumstances? Is this a good practice that isn't?

Answer 1

Contrary to some answers, there are differences in the two practices.

If you put the implementation in a class declaration, the method will automatically be marked as inline. For very short methods like wrappers this is a very useful feature .

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// test.h
class A; // forward declaration of A
// #include "decl_of_class_A.h" // not needed here, because ...

class B {
public:
  A method1();        // in these cases the forward
  void method2(A* a); // declaration of A above 
  void method3(A& a); // is completely sufficient.
};

// test.cc
#include "decl_of_class_A.h" // here we really need
                             // the complete declaration of A
                             // in order to be able to *use* it

A B::method1() { /*...*/ }
// ...

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Point p(10, 20);
Point q(0, 0);
q = p; // no assignment operator provided ==> error!

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Answer 2

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Answer 3

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class Point {
  Point(int x, int y) : x_(x), y_(y) { }
private:
  const int x_;
  const int y_;
};

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