菱形继承
Left和Right值对象在一个完整的Bottom对象内部共享着一个Top对象
使Top成为Left和Right的一个虚基类
//: C09:VirtualBase.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Shows a shared subobject via a virtual base.
#include <iostream>
using namespace std;
class Top {
protected:
int x;
public:
Top(int n) { x = n; }
virtual ~Top() {}
friend ostream&
operator<<(ostream& os, const Top& t) {
return os << t.x;
}
};
class Left : virtual public Top {
protected:
int y;
public:
Left(int m, int n) : Top(m) { y = n; }
};
class Right : virtual public Top {
protected:
int z;
public:
Right(int m, int n) : Top(m) { z = n; }
};
class Bottom : public Left, public Right {
int w;
public:
Bottom(int i, int j, int k, int m)
: Top(i), Left(0, j), Right(0, k) { w = m; }
friend ostream&
operator<<(ostream& os, const Bottom& b) {
return os << b.x << ',' << b.y << ',' << b.z
<< ',' << b.w;
}
};
int main() {
Bottom b(1, 2, 3, 4);
cout << sizeof b << endl;
cout << b << endl;
cout << static_cast<void*>(&b) << endl;
Top* p = static_cast<Top*>(&b);
cout << *p << endl;
cout << static_cast<void*>(p) << endl;
cout << dynamic_cast<void*>(p) << endl;
getchar();
} ///:~
输出
28
1,2,3,4
00FFFE30
1
00FFFE44
00FFFE30
给定类型的各个虚基类都涉及相同的对象
不论它在层次结构的哪个地方出现
打印各子对象的工作分配到各个相应的类进行
在需要的时候让派生类调用它的基类函数
尝试使用operator<<()
//: C09:VirtualBase2.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// How NOT to implement operator<<.
#include <iostream>
using namespace std;
class Top {
int x;
public:
Top(int n) { x = n; }
virtual ~Top() {}
friend ostream& operator<<(ostream& os, const Top& t) {
return os << t.x;
}
};
class Left : virtual public Top {
int y;
public:
Left(int m, int n) : Top(m) { y = n; }
friend ostream& operator<<(ostream& os, const Left& l) {
return os << static_cast<const Top&>(l) << ',' << l.y;
}
};
class Right : virtual public Top {
int z;
public:
Right(int m, int n) : Top(m) { z = n; }
friend ostream& operator<<(ostream& os, const Right& r) {
return os << static_cast<const Top&>(r) << ',' << r.z;
}
};
class Bottom : public Left, public Right {
int w;
public:
Bottom(int i, int j, int k, int m)
: Top(i), Left(0, j), Right(0, k) { w = m; }
friend ostream& operator<<(ostream& os, const Bottom& b){
return os << static_cast<const Left&>(b)
<< ',' << static_cast<const Right&>(b)
<< ',' << b.w;
}
};
int main() {
Bottom b(1, 2, 3, 4);
cout << b << endl; // 1,2,1,3,4
getchar();
} ///:~输出
1,2,1,3,4
在通常处理方式中不能盲目地向上分摊责任
因为Left和Right每个流插入程序都调用了Top流插入程序
这里需要模仿编译器的初始化方法
在类中提供特殊的函数
这种方式知道有关虚基类的情况
打印输出时忽略虚基类
//: C09:VirtualBase3.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// A correct stream inserter.
#include <iostream>
using namespace std;
class Top {
int x;
public:
Top(int n) { x = n; }
virtual ~Top() {}
friend ostream& operator<<(ostream& os, const Top& t) {
return os << t.x;
}
};
class Left : virtual public Top {
int y;
protected:
void specialPrint(ostream& os) const {
// Only print Left's part
os << ','<< y;
}
public:
Left(int m, int n) : Top(m) { y = n; }
friend ostream& operator<<(ostream& os, const Left& l) {
return os << static_cast<const Top&>(l) << ',' << l.y;
}
};
class Right : virtual public Top {
int z;
protected:
void specialPrint(ostream& os) const {
// Only print Right's part
os << ','<< z;
}
public:
Right(int m, int n) : Top(m) { z = n; }
friend ostream& operator<<(ostream& os, const Right& r) {
return os << static_cast<const Top&>(r) << ',' << r.z;
}
};
class Bottom : public Left, public Right {
int w;
public:
Bottom(int i, int j, int k, int m)
: Top(i), Left(0, j), Right(0, k) { w = m; }
friend ostream& operator<<(ostream& os, const Bottom& b){
os << static_cast<const Top&>(b);
b.Left::specialPrint(os);
b.Right::specialPrint(os);
return os << ',' << b.w;
}
};
int main() {
Bottom b(1, 2, 3, 4);
cout << b << endl; // 1,2,3,4
getchar();
} ///:~输出
1,2,3,4
specialPrint()函数是protected
因为它们只能被Bottom调用
specialPrint()函数只输出自己的数据并忽略Top子对象
//: C09:VirtInit.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Illustrates initialization order with virtual bases.
#include <iostream>
#include <string>
using namespace std;
class M {
public:
M(const string& s) { cout << "M " << s << endl; }
};
class A {
M m;
public:
A(const string& s) : m("in A") {
cout << "A " << s << endl;
}
virtual ~A() {}
};
class B {
M m;
public:
B(const string& s) : m("in B") {
cout << "B " << s << endl;
}
virtual ~B() {}
};
class C {
M m;
public:
C(const string& s) : m("in C") {
cout << "C " << s << endl;
}
virtual ~C() {}
};
class D {
M m;
public:
D(const string& s) : m("in D") {
cout << "D " << s << endl;
}
virtual ~D() {}
};
class E : public A, virtual public B, virtual public C {
M m;
public:
E(const string& s) : A("from E"), B("from E"),
C("from E"), m("in E") {
cout << "E " << s << endl;
}
};
class F : virtual public B, virtual public C, public D {
M m;
public:
F(const string& s) : B("from F"), C("from F"),
D("from F"), m("in F") {
cout << "F " << s << endl;
}
};
class G : public E, public F {
M m;
public:
G(const string& s) : B("from G"), C("from G"),
E("from G"), F("from G"), m("in G") {
cout << "G " << s << endl;
}
};
int main() {
G g("from main");
getchar();
} ///:~输出
M in B
B from G
M in C
C from G
M in A
A from E
M in E
E from G
M in D
D from F
M in F
F from G
M in G
G from main
子对象初始化顺序
所有虚基类子对象 按照它们在类定义中出现的位置 从上到下 从左到右初始化
非虚基类按通常顺序初始化
所有的成员对象按声明的顺序初始化
完整的对象构造函数执行
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