【c++】動態綁定

C++的函數調用默認不使用動態綁定。要觸發動態綁定，必須滿足兩個條件：

1. 只有指定為虛函數的成員函數才能進行動態綁定
2. 必須通過基類類型的引用或指針進行函數調用

因為每個派生類對象中都擁有基類部分，所以可以使用基類類型的指針或引用來引用派生類對象

示例

#include <iostream>
#include <string>
using namespace std;

struct base
{
    base(string str = "Base") : basename(str) {}
    virtual void print() { cout << basename << endl; }
    private:
        string basename;
};
struct derived : public base
{
    derived(string str = "Derived") : derivedname(str) {}
    void print() { cout << derivedname << endl; }
    private:
        string  derivedname;
};

int main()
{
    base b;
    derived d;
    cout << "b.print(), d.print()" << endl;
    b.print();
    d.print();

    base *pb = &b;
    base *pd = &d;
    cout << "pb->print(), pd->print()" << endl;
    pb->print();
    pd->print();

    base &yb = b;
    base &yd = d;
    cout << "yb.print(), yd.print()" << endl;
    yb.print();
    yd.print();
}

結果

 分析

可以看出基類類型的指針或引用來引用派生類對象時，調用的是重定義的虛函數。要想覆蓋虛函數機制，調用基函數的版本，可以使用強制措施。例：

代碼

#include <iostream>
#include <string>
using namespace std;

struct base
{
    base(string str = "Base") : basename(str) {}
    virtual void print() { cout << basename << endl; }
    private:
        string basename;
};
struct derived : public base
{
    derived(string str = "Derived") : derivedname(str) {}
    void print() { cout << derivedname << endl; }
    private:
        string  derivedname;
};

int main()
{
    base b;
    derived d;
    cout << "b.print(), d.print()" << endl;
    b.print();
    d.base::print();

    base *pb = &b;
    base *pd = &d;
    cout << "pb->print(), pd->print()" << endl;
    pb->print();
    pd->base::print();

    base &yb = b;
    base &yd = d;
    cout << "yb.print(), yd.print()" << endl;
    yb.print();
    yd.base::print();
}

結果