Decorator in C++
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Watch Decorator's video tutorialBefore and after
BEFORE - Inheritance run amok.
class A {
public:
virtual void do_it() {
cout << 'A';
}
};
class AwithX: public A {
public:
/*virtual*/void do_it() {
A::do_it();
do_X();
};
private:
void do_X() {
cout << 'X';
}
};
class AwithY: public A {
public:
/*virtual*/void do_it() {
A::do_it();
do_Y();
}
protected:
void do_Y() {
cout << 'Y';
}
};
class AwithZ: public A {
public:
/*virtual*/void do_it() {
A::do_it();
do_Z();
}
protected:
void do_Z() {
cout << 'Z';
}
};
class AwithXY: public AwithX, public AwithY
{
public:
/*virtual*/void do_it() {
AwithX::do_it();
AwithY::do_Y();
}
};
class AwithXYZ: public AwithX, public AwithY, public AwithZ
{
public:
/*virtual*/void do_it() {
AwithX::do_it();
AwithY::do_Y();
AwithZ::do_Z();
}
};
int main(void) {
AwithX anX;
AwithXY anXY;
AwithXYZ anXYZ;
anX.do_it();
cout << '\n';
anXY.do_it();
cout << '\n';
anXYZ.do_it();
cout << '\n';
}
AX
AXY
AXYZ
AFTER - Replacing inheritance with wrapping-delegation Discussion. Use aggregation instead of inheritance to implement embellishments to a "core" object. Client can dynamically compose permutations, instead of the architect statically wielding multiple inheritance.
class I {
public:
virtual ~I(){}
virtual void do_it() = 0;
};
class A: public I {
public:
~A() {
cout << "A dtor" << '\n';
}
/*virtual*/void do_it() {
cout << 'A';
}
};
class D: public I {
public:
D(I *inner) {
m_wrappee = inner;
}
~D() {
delete m_wrappee;
}
/*virtual*/void do_it() {
m_wrappee->do_it();
}
private:
I *m_wrappee;
};
class X: public D {
public:
X(I *core): D(core){}
~X() {
cout << "X dtor" << " ";
}
/*virtual*/void do_it() {
D::do_it();
cout << 'X';
}
};
class Y: public D {
public:
Y(I *core): D(core){}
~Y() {
cout << "Y dtor" << " ";
}
/*virtual*/void do_it() {
D::do_it();
cout << 'Y';
}
};
class Z: public D {
public:
Z(I *core): D(core){}
~Z() {
cout << "Z dtor" << " ";
}
/*virtual*/void do_it() {
D::do_it();
cout << 'Z';
}
};
int main(void) {
I *anX = new X(new A);
I *anXY = new Y(new X(new A));
I *anXYZ = new Z(new Y(new X(new A)));
anX->do_it();
cout << '\n';
anXY->do_it();
cout << '\n';
anXYZ->do_it();
cout << '\n';
delete anX;
delete anXY;
delete anXYZ;
}
AX
AXY
AXYZ
X dtor A dtor
Y dtor X dtor A dtor
Z dtor Y dtor X dtor A dtor
