递归结构的向量有内存问题

Vector of recursive structures has memory problems

本文关键字：内存 问题 向量 结构 递归      更新时间：2023-10-16

我完全被一段简单的代码所困扰，这段代码在内存上行为异常(正如Valgrind报道的那样(。我将其简化为这个简短的测试用例：

#include <vector>
struct el
{
    el * next = nullptr;
};
class list
{
public:
    list(): tail(nullptr) {}
    void push_back()
    {
        el nw;
        m_list.push_back(nw);
        if (tail == nullptr)
            tail = &m_list.back();
        else
        {
            tail->next = &m_list.back();
            tail = tail->next;
        }
    }
private:
    std::vector<el> m_list;
    el * tail;
};
int main()
{
    list a;
    a.push_back();
    a.push_back();
    return 0;
}

我希望它创建一个包含 2 个结构的数组，第一个结构有一个指向第二个结构的指针。真正的来源在破坏时因段错误而崩溃，因此我认为这份报告很重要：

==1630== Invalid write of size 8
==1630==    at 0x400A37: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400969: main (in /home/ilya/Projects/algos/a.out)
==1630==  Address 0x5a86c80 is 0 bytes inside a block of size 8 free'd
==1630==    at 0x4C2A8DC: operator delete(void*) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1630==    by 0x4011D5: __gnu_cxx::new_allocator<el>::deallocate(el*, unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400F79: std::_Vector_base<el, std::allocator<el> >::_M_deallocate(el*, unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400DA3: void std::vector<el, std::allocator<el> >::_M_emplace_back_aux<el const&>(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400B42: std::vector<el, std::allocator<el> >::push_back(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4009FF: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400969: main (in /home/ilya/Projects/algos/a.out)
==1630==  Block was alloc'd at
==1630==    at 0x4C29780: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1630==    by 0x4012BB: __gnu_cxx::new_allocator<el>::allocate(unsigned long, void const*) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4010D2: std::_Vector_base<el, std::allocator<el> >::_M_allocate(unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400CC1: void std::vector<el, std::allocator<el> >::_M_emplace_back_aux<el const&>(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400B42: std::vector<el, std::allocator<el> >::push_back(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4009FF: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x40095D: main (in /home/ilya/Projects/algos/a.out)
==1630==
==1630== Invalid read of size 8
==1630==    at 0x400A42: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400969: main (in /home/ilya/Projects/algos/a.out)
==1630==  Address 0x5a86c80 is 0 bytes inside a block of size 8 free'd
==1630==    at 0x4C2A8DC: operator delete(void*) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1630==    by 0x4011D5: __gnu_cxx::new_allocator<el>::deallocate(el*, unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400F79: std::_Vector_base<el, std::allocator<el> >::_M_deallocate(el*, unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400DA3: void std::vector<el, std::allocator<el> >::_M_emplace_back_aux<el const&>(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400B42: std::vector<el, std::allocator<el> >::push_back(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4009FF: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400969: main (in /home/ilya/Projects/algos/a.out)
==1630==  Block was alloc'd at
==1630==    at 0x4C29780: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==1630==    by 0x4012BB: __gnu_cxx::new_allocator<el>::allocate(unsigned long, void const*) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4010D2: std::_Vector_base<el, std::allocator<el> >::_M_allocate(unsigned long) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400CC1: void std::vector<el, std::allocator<el> >::_M_emplace_back_aux<el const&>(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x400B42: std::vector<el, std::allocator<el> >::push_back(el const&) (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x4009FF: list::push_back() (in /home/ilya/Projects/algos/a.out)
==1630==    by 0x40095D: main (in /home/ilya/Projects/algos/a.out)

一旦您push_back向量或以其他方式更改它，所有指向存储项目的引用和指针都将失效，因此所有tail和next字段都将包含悬空指针。取消引用它们将导致未定义的行为。您实际上可以使用std::list或者(如果您只是出于学习目的编写一些列表实现(，您可以首先填充向量，然后收集指向存储项的指针，知道它们将保持有效。

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