c++ - 为什么我收到错误: “there is more than one default constructor” ?

Question

我收到消息:

Severity Code Description Project File Line Suppression State Error (active) E0339 class "D" has more than one default constructor)

和:

Severity Code Description Project File Line Suppression State Error C2668 'D::D': ambiguous call to overloaded function)

错误发生在标记为//(2)的行中

如果删除标记///(1)的行，则可以构建代码。

class C {
    int i, j;

public:
    C(int x, int y) : i(x), j(y)
    {
        cout << "Konstr C" << endl;
    }
    C() : i(0), j(0)
    {
        cout << "Std-Konstr C" << endl;
    }
        ~C()
    {
        cout << "Destruktor C" << endl;
    }
};
class D : public C {
    int k, a, b;
    C c;
public:

    D():c(){ cout << "Std-Konstr D" << endl; }// (1)

    D(int x = 1) :c(x, 1), a(x), b(0), k(19)

    {
        cout << "Konstr-1 D" << endl;
    }
    D(int x, int y, int z) :C(x, y), a(1), b(2), c(x, y), k(z)
    {
        cout << "Konstr-2 D" << endl;
    }
    ~D()
    {
        cout << "Destruktor D" << endl;
    }
};
class E : public D {
    int m;
    C c;
    D b;
public:
    E(int x, int y) : c(2, 3), b(y), m(x + y)// (2)
    {
        cout << "Konstr E" << endl;
    }
    ~E()
    {
        cout << "Destruktor E" << endl;
    }
};

Answer 1

如错误消息所述，D()是不明确的。编译器无法知道是要调用no-arg构造函数，还是要使用默认值int调用1构造函数。

消除这种歧义的一种方法是删除x参数的默认值:

D():c(){ cout << "Std-Konstr D" << endl; }// (1)

D(int x) :c(x, 1), a(x), b(0), k(19)
//    ^-- x=1 was removed here
{
    cout << "Konstr-1 D" << endl;
}