C++ 接口(interface)、继承、多态性

Question

想象一下这个场景:

• 接口(interface)I1:m1()，由C1类实现
• 接口(interface)I2:m2()，由C2类实现
• 接口(interface)I3:m3()，由C3类实现

我想定义接受参数 arg 的函数。

void f1(I1 arg) {
    use m1()
}

void f2([I1, I2] arg) {
    use m1() and m2()
}

void f3([I2, I3] arg) {
    use m2() and m3()
}

然后我要定义:

• 一个“union ”接口(interface) I123，它具有所有方法，由 C123
实现
• 一个“union ”类C123，它继承现有类C1、C2、C3 的实现方法。

然后我想实例化C123并将其与f1、f2、f3一起使用。

C123 obj;

f1(obj);
f2(obj);
f3(obj);

问题

1. 这在 C++ 中可行吗？
2. 没有接口(interface)是否可行？例如。函数 f2 将采用 [C1, C2] 而不是 [I1, I2]。

---

代码

#include <string>
#include <iostream>

using namespace std;

class C1 {
protected:
  int i;
public:
  int getI() const { return i; }
  void setI(int i_) { i = i_; }
};

class C2 {
protected:
  string s;
public:
  string getS() const { return s; }
  void setS(string s_) { s = s_; }
};

class C3 {
protected:
  float f;
public:
  float getF() const { return f; }
  void setF(float f_) { f = f_; }
};

class C23 : public C2, public C3 {};
class C123 : public C1, public C2, public C3 {};

void f3(C23 arg) {
  arg.setS(to_string(arg.getF()));
}


int main() {

  C123 obj;

  f3(obj);

  std::cout << obj.getS();
}

错误信息

a.cc: In function ‘int main()’:
a.cc:42:9: error: could not convert ‘obj’ from ‘C123’ to ‘C23’
   f3(obj);

Answer 1

1. Is this possible in C++?

是的，可以用 C++ 实现。

Example solution using interfaces

#include <string>
#include <iostream>

using namespace std;

class I1 {
public:
    virtual int getI() const = 0;
    virtual void setI(int i) = 0;
};

class I2 {
public:
    virtual string getS() const = 0;
    virtual void setS(string s) = 0;
};

class I3 {
public:
    virtual float getF() const = 0;
    virtual void setF(float f) = 0;
};

class C1 : public I1 {
protected:
    int i;
public:
    int getI() const { return i; }
    void setI(int i_) { i = i_; }
};

class C12 : public I1, public I2 {
protected:
    int i;
    string s;
public:
    int getI() const { return i; }
    void setI(int i_) { i = i_; }
    string getS() const { return s; }
    void setS(string s_) { s = s_; }
};

class C123 : public I1, public I2, public I3 {
protected:
    int i;
    string s;
    float f;
public:
    int getI() const { return i; }
    void setI(int i_) { i = i_; }
    string getS() const { return s; }
    void setS(string s_) { s = s_; }
    float getF() const { return f; }
    void setF(float f_) { f = f_; }
};

template<class T>
void f1(const T& c1)
{
    cout << "f1:\n";
    cout << "  getI: " << c1.getI() << endl;
}

template<class T>
void f2(const T& c12)
{
    cout << "f2:\n";
    cout << "  getI: " << c12.getI() << endl;
    cout << "  getS: " << c12.getS() << endl;
}

template<class T>
void f3(const T& c23)
{
    cout << "f3:\n";
    cout << "  getS: " << c23.getS() << endl;
    cout << "  getF: " << c23.getF() << endl;
}

void test()
{
    C1 c1;
    c1.setI(1);
    f1(c1);

    cout << "\n===== " << endl;

    C12 c12;
    c12.setI(12);
    c12.setS("str12");
    f1(c12);
    f2(c12);

    cout << "\n===== " << endl;

    C123 c123;
    c123.setI(123);
    c123.setF(1.23f);
    c123.setS("str123");
    f1(c123);
    f2(c123);
    f3(c123);

    cout << "\n===== " << endl;
}

int main()
{
    test();
}

2. Is it possible without interfaces? Eg. function f2 would take [C1, C2] instead of [I1, I2].

是的，没有接口(interface)也可以做到这一点。

Solution using virtual inheritance (without using interfaces) :

#include <string>
#include <iostream>

using namespace std;

class C1 {
protected:
    int i;
public:
    int getI() const { return i; }
    void setI(int i_) { i = i_; }
};

class C2 {
protected:
    string s;
public:
    string getS() const { return s; }
    void setS(string s_) { s = s_; }
};

class C3 {
protected:
    float f;
public:
    float getF() const { return f; }
    void setF(float f_) { f = f_; }
};

class C12 : public virtual C1, public virtual C2
{
};

class C23 : public virtual C2, public virtual C3
{
};

class C123 : public virtual C1, public virtual C12, public virtual C23
{
};


void f1(const C1& c1)
{
    cout << "f1:\n";
    cout << "  getI: " << c1.getI() << endl;
}

void f2(const C12& c12)
{
    cout << "f2:\n";
    cout << "  getI: " << c12.getI() << endl;
    cout << "  getS: " << c12.getS() << endl;
}

void f3(const C23& c23)
{
    cout << "f3:\n";
    cout << "  getS: " << c23.getS() << endl;
    cout << "  getF: " << c23.getF() << endl;
}

void test()
{
    C1 c1;
    c1.setI(1);
    f1(c1);

    cout << "\n===== " << endl;

    C12 c12;
    c12.setI(12);
    c12.setS("str12");
    f1(c12);
    f2(c12);

    cout << "\n===== " << endl;

    C123 c123;
    c123.setI(123);
    c123.setF(1.23f);
    c123.setS("str123");
    f1(c123);
    f2(c123);
    f3(c123);

    cout << "\n===== " << endl;
}

int main()
{
    test();
}

一些解释。如果你需要你的 f2 只接受 C1 和 C2(没有 C3)，那么它必须是在 C1 和 C2 上都继承的独立类型。 f3 也一样。然后，如果您遵循该逻辑并在不使用虚拟继承的情况下创建类 C12 和 C23，那么您现在应该从 C12 和 C23 继承的 C123 最终将具有多个拷贝C2，当您调用 f2 和 f3 时，您会从 getS 获得不同的值。虚拟继承确保在继承层次结构中只有一个类的拷贝。

a "union" class C123 which inherits implemented methods from existing classes C1, C2, C3

Solution that use virtual inheritance and interfaces

#include <string>
#include <iostream>

using namespace std;

class I1 {
public:
    virtual int getI() const = 0;
    virtual void setI(int i) = 0;
};

class I2 {
public:
    virtual string getS() const = 0;
    virtual void setS(string s) = 0;
};

class I3 {
public:
    virtual float getF() const = 0;
    virtual void setF(float f) = 0;
};

class I12 : virtual public I1, virtual public I2 {};
class I23 : virtual public I2, virtual public I3 {};
class I123 : virtual public I12, virtual public I23 {};

class C1 : virtual public I1 {
protected:
    int i;
public:
    int getI() const { return i; }
    void setI(int i_) { i = i_; }
};

class C2 : virtual public I2 {
protected:
    string s;
public:
    string getS() const { return s; }
    void setS(string s_) { s = s_; }
};

class C3 : virtual public I3 {
protected:
    float f;
public:
    float getF() const { return f; }
    void setF(float f_) { f = f_; }
};

class C12 : public I12, public C1, public C2 {};
class C123 : public I123, public C1, public C2, public C3 {};

void f1(const I1& c1)
{
    cout << "f1:\n";
    cout << "  getI: " << c1.getI() << endl;
}

void f2(const I12& c12)
{
    cout << "f2:\n";
    cout << "  getI: " << c12.getI() << endl;
    cout << "  getS: " << c12.getS() << endl;
}

void f3(const I123& c23)
{
    cout << "f3:\n";
    cout << "  getS: " << c23.getS() << endl;
    cout << "  getF: " << c23.getF() << endl;
}

void test()
{
    C1 c1;
    c1.setI(1);
    f1(c1);

    cout << "\n===== " << endl;

    C12 c12;
    c12.setI(12);
    c12.setS("str12");
    f1(c12);
    f2(c12);

    cout << "\n===== " << endl;

    C123 c123;
    c123.setI(123);
    c123.setF(1.23f);
    c123.setS("str123");
    f1(c123);
    f2(c123);
    f3(c123);

    cout << "\n===== " << endl;
}

int main()
{
    test();
}

所有解决方案都应产生此输出:

f1:
  getI: 1

=====
f1:
  getI: 12
f2:
  getI: 12
  getS: str12

=====
f1:
  getI: 123
f2:
  getI: 123
  getS: str123
f3:
  getS: str123
  getF: 1.23

=====