C++ - 实例化派生类并使用基类的构造函数

Question

我有一个基类,其构造函数需要一个参数(字符串).然后我有一个派生类,它也有自己的构造函数.我想实例化派生类,并且能够设置基类的构造函数的参数.

class BaseClass {
    public:
        BaseClass (string a);
};

class DerivedClass : public BaseClass {
    public:
        DerivedClass (string b);
};

int main() {
    DerivedClass abc ("Hello");
}

我不确定在调用派生类时如何设置基类构造函数的参数.

Answer 1

你有两种可能性 - 内联:

class DerivedClass : public BaseClass {
public:
    DerivedClass (string b) : BaseClass(b) {}
};

或超出范围:

class DerivedClass : public BaseClass {
public:
    DerivedClass (string b);
};

/* ... */
DerivedClass::DerivedClass(string b) : BaseClass(b)
{}

更多例子:

class DerivedClass : public BaseClass {
public:
    DerivedClass(int a, string b, string c);

private:
    int x;
};

DerivedClass::DerivedClass(int a, string b, string c) : BaseClass(b + c), x(a)
{}

在初始化列表上:

class MyType {
public:
    MyType(int val) { myVal = val; }    // needs int
private:
    int myVal;
};

class DerivedClass : public BaseClass {
public:
    DerivedClass(int a, string b) : BaseClass(b)
    {  x = a;  }   // error, this tries to assign 'a' to default-constructed 'x'
                   // but MyType doesn't have default constructor

    DerivedClass(int a, string b) : BaseClass(b), x(a)
    {}             // this is the way to do it
private:
    MyType x;
};