Why I get the error: "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)

错误发生在标有 //(2)

的行

如果我删除标有 //(1) 的行，我可以构建我的代码。

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() 不明确。编译器无法知道您是要调用 no-arg 构造函数，还是调用默认值为 1.

的 int 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;
}

Why I get the error: "there is more than one default constructor"?

Why I get the error: "there is more than one default constructor"?

c++

constructor

compiler-errors

default-constructor

c++17