c++ - 在类中获取指向自身的指针
问题描述
如何在类中获取指向自身对象的指针?下面的简单代码显示了两个对象的创建:A 和 B。A 包含指向 B 的指针,我必须在 B 的构造函数中分配它。
#include<iostream>
using namespace std;
class B;
class A {
public:
B* ptr;
};
class B {
char array[1024];
public:
int value = 12;
B() {};
B(A* a) {
a->ptr = this;
cout << "constructor B addr = " << reinterpret_cast<std::uintptr_t>(this) << endl;
}
};
int main() {
B b;
A a;
b = B(&a);
cout << "stack B addr = " << reinterpret_cast<std::uintptr_t>(&b) << endl;
a.ptr->value = 10;
cout << "B.value = " << b.value << endl;
return 0;
};
此代码的示例输出似乎在逻辑上不正确:
constructor B addr = 140737243484752
stack B addr = 140737243483712
B.value = 12
那么,是否有任何变体可以获取指向自身内部对象的指针?
解决方案
问题是您正在创建多个B
实例。第一个实例位于 : 的main
开头B b;
。几行之后,您构造另一个 B
( B(&a)
),然后将其分配给b
. 您的输出是正确的,因为constructor B addr
是从临时B
的构造函数输出的,而不是B
您实际上保留在main
.
通过消除临时性,我得到了您似乎期望的输出:
int main() {
A a;
B b(&a);
cout << "stack B addr = " << reinterpret_cast<std::uintptr_t>(&b) << endl;
a.ptr->value = 10;
cout << "B.value = " << b.value << endl;
return 0;
};
可能的输出(地址显然会不同)。
constructor B addr = 140731920033760
stack B addr = 140731920033760
B.value = 10
此外,从技术上讲,您在原始代码中的写入无效,因为a.ptr
具有临时地址。如果您使用地址清理程序进行编译,您将获得一些有用的输出:
constructor B addr = 140732509397056
stack B addr = 140732509395968
=================================================================
==3080==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffed73b2440 at pc 0x55ae5d0dc4c2 bp 0x7ffed73b1b90 sp 0x7ffed73b1b80
WRITE of size 4 at 0x7ffed73b2440 thread T0
#0 0x55ae5d0dc4c1 in main /tmp/so/addr.cpp:32
#1 0x7f2cdc4d0ae6 in __libc_start_main (/lib64/libc.so.6+0x21ae6)
#2 0x55ae5d0dc149 in _start (/tmp/so/add+0x1149)
Address 0x7ffed73b2440 is located in stack of thread T0 at offset 2208 in frame
#0 0x55ae5d0dc239 in main /tmp/so/addr.cpp:25
This frame has 3 object(s):
[32, 40) 'a'
[96, 1124) 'b'
[1184, 2212) '<unknown>' <== Memory access at offset 2208 is inside this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
(longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope /tmp/so/addr.cpp:32 in main
Shadow bytes around the buggy address:
0x10005ae6e430: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
0x10005ae6e440: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
0x10005ae6e450: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
0x10005ae6e460: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
0x10005ae6e470: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
=>0x10005ae6e480: f8 f8 f8 f8 f8 f8 f8 f8[f8]f2 f2 f2 00 00 00 00
0x10005ae6e490: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005ae6e4a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005ae6e4b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005ae6e4c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005ae6e4d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
==3080==ABORTING
推荐阅读
- api - 如何从 github api 获取所有标签
- c# - 我想使用 c# 将控制台中的文本水平和垂直居中
- excel - 无法在后台杀死excel应用程序
- javascript - Kubernetes 内的 Express API,用于反应和反应原生应用程序
- gradle - 如何在 build.gradle 中嵌入 gradle 的共同价值
- xml - 如何使用powershell循环创建XML文件的元素?
- python - 我无法安装 python 包(opencv)
- azure - 是否可以使用 VSTS/Azure DevOps Release 部署任务对 Azure 功能进行故障转移?
- javascript - 在 Django 中使用 JavaScript 提交表单不起作用
- angular - Azure 应用服务上 Angular 应用的应用程序错误