C++类成员中包含其他类时的初始化问题

当B中包含A类时，A在什么时候初始化呢？

例如：

#include<iostream>
using namespace std;
class A
{
public:
A()
{
cout << "A is constructed!" << endl;
}
};
class B
{
private:
A a;
public:
B()
{
a = A();
cout << "B is constructed!" << endl;
}
};
int main()
{
B b = B{};

system("pause");
}

A是在声明时构造还是在B()中构造呢？运行结果如下

A is constructed!

A is constructed!

B is constructed!

A构造了两次，说明A在声明时就已经调用了构造函数。

如何只让A调用一次呢？可以使用指针解决这个问题。在A的构造函数中带有参数时，会给参数传递带来方便。

#include<iostream>
using namespace std;
class A
{
public:
A()
{
cout << "A is constructed!" << endl;
}
};
class B
{
private:
A *a;
public:
B()
{
a = new A();
cout << "B is constructed!" << endl;
}
~B()
{
delete a;
}
};
int main()
{
B b = B{};
system("pause");
}

运行结果如下：

A is constructed!

B is constructed!