c++ - 为什么使用指针而不是直接实例化对象时不使用类成员的默认值？

Question

在下面的代码中:

#include<iostream>
using namespace std;

class Father
{
public:
    int a=11;
  void f(){ cout<<"f called"<<endl;}
};


int main(){

  Father *obj;
  cout <<obj->a<<endl; //I get a garbage value and the compiler issues a warning: 'obj' is used uninitialized in this function

  Father f;
  cout <<f.a<<endl; // it prints 11
return 1;
}

cout <<obj->a<<endl;正在打印垃圾值，而不是默认值11，就像上面的一样。他们不是应该打印相同的吗？
为什么使用指针而不是直接实例化对象时不使用类成员的默认值？

Answer 1

Aren't they suppose to print the same?

不，这是 UB。

给定 Father *obj;， obj是 default-initialized，其值不确定，

Default initialization of non-class variables with automatic and dynamic storage duration produces objects with indeterminate values

这意味着 obj没有指向任何有效的对象，并且对其取消引用会导致 undefined behavior，一切皆有可能。您需要使其指向有效对象，例如

Father f;
Father *obj = &f;
cout <<obj->a<<endl;

要么

Father *obj = new Father;
cout <<obj->a<<endl;
delete obj;

Father f;也执行 default-initialization。作为类类型， f.a初始化为值 11。

if T is a non-POD (until C++11) class type, the constructors are considered and subjected to overload resolution against the empty argument list. The constructor selected (which is one of the default constructors) is called to provide the initial value for the new object;

顺便说一句:C++ 11支持 Default member initializer(在 int a=11;类中编写 Father时)，请尝试使用C++ 11(或更高版本)模式编译代码。