从地图中删除指针 [英] Delete pointers from a map

问题描述

有一张地图，它将 int 映射到测试* 。

所有 Test * 指针在之前分配并稍后分配给映射。然后，我删除 的值，并将它们设置为 null 。 / p>

之后，它检查一个的有效性，它应该是 null 。 但，一个不是 null 。

  #include< QString> 
 #include< QMap> 
 #include< QDebug> 
 
 class Test {
 QString name; 
 public：
 Test（const QString& name）：name（name）{} 
 QString getName（）const {return name; } 
}; 
 
 int main（）{
 QMap< int，Test *>地图; 
 
 Test * one = new Test（one）; 
 Test * two = new Test（two）; 
 Test * three = new Test（three）; 
 
 map.insert（1，one）; 
 map.insert（2，two）; 
 map.insert（3，three）; 
 
 for（auto itr = map.begin（）; itr！= map.end（）; itr ++）{
 Test * x = * itr; 
 if（x）{
 delete x; 
 x = 0; // **设置null为指针** // 
} 
} 
 
 if（one）// **这里一个不是0？ ** // 
 qDebug（）<< one-> getName（）<< endl; // **然后这里崩溃** // 
} 
  

如何可以使用

它是否固定？

第二个问题是，是否正确 delete ？

解决方案

在循环中，变量 x 只在循环内。当你将 NULL 设置为 NULL 。

你应该设置通过解引用迭代器返回的引用为 NULL ：

  * itr = nullptr; 
  

这将使地图中的指针 NULL



---

当你有两个指针时，它类似这样：

 
 + ----- + 
 |一| -  \ 
 + ----- + | + --------------- + 
>  - > |测试实例| 
 + ----- + | + --------------- + 
 | x | --- / 
 + ----- + 
 

如果您设置了指针它看起来像这样：

 
 + ----- + 
 |一| --- \ 
 + ----- + | + --------------- + 
>  - > |测试实例| 
 + ----- + + --------------- + 
 | x | 
 + ----- + 
 

变量 x NULL 但是变量一个仍指向对象。如果对象已删除，则取消引用该指针，将导致未定义的行为

。

There is a map, it maps an int to a Test*.

All Test* pointers are allocated before and assigned to the map later. Then, I'm deleteing values of map and set them to null.

After that, It checks validity of one, and it supposed to be null. But, one is not null.

#include <QString>
#include <QMap>
#include <QDebug>

class Test {
    QString name;
public:
    Test(const QString &name) : name(name) {}
    QString getName() const { return name; }
};

int main() {
    QMap<int, Test*> map;

    Test *one = new Test("one");
    Test *two = new Test("two");
    Test *three = new Test("three");

    map.insert(1, one);
    map.insert(2, two);
    map.insert(3, three);

    for (auto itr = map.begin(); itr != map.end(); itr++) {
        Test *x = *itr;
        if (x) {
            delete x;
            x = 0; // ** Sets null to the pointer ** //
        }
    }

    if (one) // ** Here one is not 0 ?! ** //
        qDebug() << one->getName() << endl; // ** And then here crashes ** //
}

I think, I missed something when I'm deleteing them in the loop.

How can it fixed?

Second question is, is it correctly deletes the allocated pointers?

解决方案

In the loop, the variable x is a local pointer only inside the loop. When you set that to NULL you don't actually set any other pointers to NULL.

What you should to is to set the reference returned by dereferencing the iterator to NULL:

*itr = nullptr;

This will make the pointer in the map be NULL, but the other pointers will still be pointing to a now deallocated memory area.

---

When you have two pointers, it kind of looks like this:

+-----+
| one | ---\
+-----+     |     +---------------+
             >--> | Test instance |
+-----+     |     +---------------+
|  x  | ---/
+-----+

If you set one of the pointers it looks like this:

+-----+
| one | ---\
+-----+     |     +---------------+
             >--> | Test instance |
+-----+           +---------------+
|  x  | 
+-----+

The variable x is NULL but the variable one still points to the object. And if the object has been deleted then dereferencing that pointer you will cause undefined behavior.

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