c++ - 大小为 8 的 Valgrind 无效读取 C++

Question

Valgrind 中的这个错误是什么意思？我看了很多其他帖子，但我仍然不明白错误的实际含义。我的 Dragon 析构函数有问题吗？

==48500== Invalid read of size 8
==48500==    at 0x40AF66: Dragon::~Dragon() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x40AFDB: Dragon::~Dragon() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x406841: Floor::deleteAll() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x4022D8: Floor::~Floor() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x401EE1: main (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==  Address 0x5a1d130 is 0 bytes inside a block of size 40 free'd
==48500==    at 0x4C2A4BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==48500==    by 0x40A7B3: Treasure::~Treasure() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x40671B: Floor::deleteAll() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x4022D8: Floor::~Floor() (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)
==48500==    by 0x401EE1: main (in /u3/nsah/cs246/1149/a5/cc3k/cc3k/test)

编辑:这就是我的 Dragon 类的样子:

#include "dragon.h"
#include <iostream>
#include <cmath>
#include <cstdlib>

using namespace std;

Dragon::Dragon(Item *hoard) {
    // initalizes some variables
    treasureHoard = hoard;
}

Dragon::~Dragon(){
    treasureHoard->deadDragon();
}

// some functions

编辑:基本上我的代码是这样做的:

D *ptr=new D();
// some code …
if (ptr->isDead()) {
    delete ptr;
    ptr = NULL;
}
// some code

// at the end of the program: 
if (ptr) {   // <<< is this causing the error?
    delete ptr;
    ptr = NULL;
}

Answer 1

我认为 valgrind 消息非常清楚:

==48500==  Address 0x5a1d130 is 0 bytes inside a block of size 40 free'd

有一个数据 block 被“释放”(在 C++ 中很可能实际被“删除”)并且仍在被访问。更准确地说，正在读取大小为 40 的已释放内存块开头的一个 8 字节字。

也就是说，您有效地让代码执行与此等同的道德操作:

T* ptr = new T; // allocator memory of an object of size 40
...
delete ptr;           // release the memory
...
X value = ptr->start; // read the first word of the data

(很明显，类型、成员和变量名称在代码中会有所不同，但大致就是这样)

这是一个复制问题的完整程序(尽管堆栈更简单):

struct foo {
    double values[5];
};

int main()
{
    foo* ptr = new foo();
    delete ptr;
    double x = ptr->values[0];
    return x;
}