c++继承问题

[zzh193]

请高手指教以下程序错在哪？
#include<iostream>
using namespace std;
class A
{
int x;
public:
A(int a):x(a){cout<<"Constructing A\n";}
};
class B:public A
{
public:
B(){cout<<"Constructing B\n";}
};
int main()
{
B b;
}

[qqwx_1986]

加个
A():x(0){cout < <"Constructing A\n";}

[sagegz]

没有合适的默认构造函数.

[华亭真人]

当实例化B时，会调用B的默认构造函数,因为没有参数，就会试图调用A的默认构造函数...但A没有,这样就出错了

[jichre]

#include <iostream>
using namespace std;
class A
{
int x;
public:
A(int a=0):x(a){cout < <"Constructing A\n";} //提供默认构造A(int a=0)
};
class B:public A
{
public:
B(){cout < <"Constructing B\n";}
};
int main()
{
B b;
}