为什么这段代码只打印42？ [英] Why does this code only print 42?

问题描述

有人请解释一下为什么这段代码只打印42而不是created\\\
42？

Could somebody please explain to me why does this code only print "42" instead of "created\n42"?

#include <iostream>
#include <string>
#include <memory>

using namespace std;

class MyClass
{
public:
    MyClass() {cout<<"created"<<endl;};
    int solution() {return 42;}
    virtual ~MyClass() {};
};

int main(int argc, char *argv[])
{
    auto_ptr<MyClass> ptr;
    cout<<ptr->solution()<<endl;
    return 0;
}

我试了这个代码在解决方案不同的值，我总是得到

BTW I tried this code with different values in solution and I always get the "right" value, so it doesn't seem to be a random lucky value.

推荐答案

因为它展现未定义的行为 - 你取消引用一个空指针。

Because it exhibits undefined behaviour - you dereference a null pointer.

当你说：

 auto_ptr<MyClass> ptr;

您创建一个自动指向器，不指向任何东西。这相当于说：

you create an autopointer which doesn't point to anything. This is equivalent to saying:

MyClass * ptr = NULL;

然后当您说：

cout<<ptr->solution()<<endl;

您将引用此空指针。这在C ++中是未定义的 - 对于你的实现，它似乎工作。

you dereference this null pointer. Doing that is undefined in C++ - for your implementation, it appears to work.

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