Custom Constructor

Constructors: why? (1)

Initialization in C

• left to the programmer

• ⟶ sheer number of bugs!

struct point A;	A remains uninitialized ⟶ random values
struct point A = {1,2};	A is initialized with x = 1 and y = 2
struct point A; ... A.x = 1; A.y = 2;	Definition ⟶ uninitialized Only set at some later point ⟶ error prone

Constructors: why? (2)

Initialization in C++

• Programmer has no choice

• Whenever programmer thinks about a point object, they have to think about its value

• ⟶ initialization error excluded from the beginning

point A;	Compiler error: “void constructor for point not defined”
point A(1,2);	Only possibility to create a point

Constructors: Implementation - Inline

Short methods are best defined in the class definition itself ⟶ inline

point.h: inline definition

class point
{
public:
    point(int x, int y)
    {
        _x = x;
        _y = y;
    }
private:
    int _x;
    int _y;
};

See also

• Constructors: Member Initialization

• Better: using an initializer list for member initialization

• No difference for non const members

• Assignment in constructor body not possible for const members ⟶ initializer list necessary

point.h: initializer list

class point
{
public:
    point(int x, int y)
    : _x{x}, _y{y}        // <--- initializer list
    {}                    // <--- empty constructor body
private:
    const int _x;         // <--- note the "const"
    const int _y;         // <--- note the "const"
};

Constructors: Implementation - Out-of-Line

• Long methods are best defined in the implementation file

• (class point constructor is not a long method though …)

point.h: declaration	point.cpp: definition
class point { public: point(int x, int y); };	point::point(int x, int y) // <--- note the SCOPE: "point::" : _x{x}, _y{y} {}