为什么在尝试使用地图时会出现访问违规

Why do I get access violation when trying to use a map

本文关键字：访问 地图 为什么      更新时间：2023-10-16

我这里有一个类，用来记录内存映射的位置。当我将地图上的特定位置设置为某个数字时，就会出现访问违规。

确切的错误是在memorymapperexample.exe:0xC0000005:读取位置0x00000004时发生0x003A5B4B异常。

这是我的课。Addpair和clearlocation在调用时会出现错误。map键是一个void指针，map值是一个int。我不明白为什么这个简单的东西不起作用。

#include <map>
#include <unordered_map>
#include <string>
#include <iostream>
#include <memory>
using namespace std;

template <typename T>
class MallocAllocator
{
private:
    map<void *, int> ord;
    int total;
public:
    typedef T value_type;
    MallocAllocator() { }
    template <typename U> MallocAllocator(const MallocAllocator<U>& other) {}
    T* allocate(size_t count)
    {
        return (T*)malloc(count * sizeof(T));
    }
    void deallocate(T* object, size_t n)
    {
        void* ptr = reinterpret_cast<void*>(object);
        free(ptr);
    }
    void addpair(void* a, int b)
    {
        ord[a] = b; //ERROR HERE
    }
    void clearlocation(void * a)
    {
        ord[a] = 0; //ERROR HERE TOO
    }
};
MallocAllocator<void*> memoryManager;
void* operator new(size_t size)
{
    cout << "Allocating memory..." << endl;
    auto newObject = memoryManager.allocate(size);
    memoryManager.addpair(newObject, size);
    memoryManager.clearlocation(newObject);
    return newObject;
}
void operator delete(void* objectPtr) noexcept
{
    cout << "DEAllocating memory..." << endl;
    void** ptr = reinterpret_cast<void**>(objectPtr);
    memoryManager.deallocate(ptr, 0);
    //free(objectPtr);
}

int main()
{
    int * ima = new int(99);
    delete ima;
    return 1;
}

发现您的问题。通过VS运行它给了我这个堆栈跟踪。

bleh.exe!std::_Tree<std::_Tmap_traits<void * __ptr64,int,std::less<void * __ptr64>,std::allocator<std::pair<void * __ptr64 const,int> >,0> >::_Lbound<void * __ptr64>(void * const & _Keyval) Line 2090 C++
bleh.exe!std::_Tree<std::_Tmap_traits<void * __ptr64,int,std::less<void * __ptr64>,std::allocator<std::pair<void * __ptr64 const,int> >,0> >::lower_bound(void * const & _Keyval) Line 1549 C++
bleh.exe!std::map<void * __ptr64,int,std::less<void * __ptr64>,std::allocator<std::pair<void * __ptr64 const,int> > >::_Try_emplace<void * __ptr64 const & __ptr64>(void * const & _Keyval) Line 210    C++
bleh.exe!std::map<void * __ptr64,int,std::less<void * __ptr64>,std::allocator<std::pair<void * __ptr64 const,int> > >::try_emplace<>(void * const & _Keyval) Line 230   C++
bleh.exe!std::map<void * __ptr64,int,std::less<void * __ptr64>,std::allocator<std::pair<void * __ptr64 const,int> > >::operator[](void * const & _Keyval) Line 339  C++
bleh.exe!MallocAllocator<void * __ptr64>::addpair(void * a, int b) Line 80  C++
bleh.exe!operator new(unsigned __int64 size) Line 97    C++
[External Code] 
bleh.exe!MallocAllocator<void * __ptr64>::MallocAllocator<void * __ptr64>() Line 66 C++
bleh.exe!`dynamic initializer for 'memoryManager''() Line 89    C++
[External Code]

堆栈显示，当有东西调用new时，程序正在初始化memoryManager（我猜std::map正在尝试分配一些东西）。但当然，您已经重载了new以使用尚未完全初始化的memoryManager！真倒霉它在试图获取std::map的根节点时抛出错误，该根节点没有给出合理的值。

调试是一项非常有用的技能。

编辑：使用g++ -std=c++11 bleh.cpp编译并通过gdb运行会给我一个不同但同样令人不安的结果。

#0  0x00007ffff756a648 in _IO_new_file_xsputn (f=0x7ffff78b0400 <_IO_2_1_stdout_>, data=0x402597, n=20) at fileops.c:1320
#1  0x00007ffff755fe6d in __GI__IO_fwrite (buf=<optimized out>, size=1, count=20, fp=0x7ffff78b0400 <_IO_2_1_stdout_>) at iofwrite.c:43
#2  0x00007ffff7b7f63e in std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) ()  from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007ffff7b7f947 in std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x0000000000400d11 in operator new(unsigned long) ()
#5  0x000000000040228b in __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<void* const, int> > >::allocate(unsigned long, void const*) ()
#6  0x0000000000402142 in std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<void* const, int> > > >::allocate(std::allocator<std::_Rb_tree_node<std::pair<void* const, int> > >&, unsigned long) ()
#7  0x0000000000401d75 in std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_get_node() ()
#8  0x00000000004015cd in std::_Rb_tree_node<std::pair<void* const, int> >* std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_create_node<std::piecewise_construct_t const&, std::tuple<void* const&>, std::tuple<> >(std::piecewise_construct_t const&, std::tuple<void* const&>&&, std::tuple<>&&) ()
#9  0x0000000000401368 in std::_Rb_tree_iterator<std::pair<void* const, int> > std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_emplace_hint_unique<std::piecewise_construct_t const&, std::tuple<void* const&>, std::tuple<> >(std::_Rb_tree_const_iterator<std::pair<void* const, int> >, std::piecewise_construct_t const&, std::tuple<void* const&>&&, std::tuple<>&&) ()
#10 0x0000000000401122 in std::map<void*, int, std::less<void*>, std::allocator<std::pair<void* const, int> > >::operator[](void* const&) ()
#11 0x0000000000400ee4 in MallocAllocator<void*>::addpair(void*, int) ()
#12 0x0000000000400d4a in operator new(unsigned long) ()
#13 0x000000000040228b in __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<void* const, int> > >::allocate(unsigned long, void const*) ()
#14 0x0000000000402142 in std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<void* const, int> > > >::allocate(std::allocator<std::_Rb_tree_node<std::pair<void* const, int> > >&, unsigned long) ()
#15 0x0000000000401d75 in std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_get_node() ()
#16 0x00000000004015cd in std::_Rb_tree_node<std::pair<void* const, int> >* std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_create_node<std::piecewise_construct_t const&, std::tuple<void* const&>, std::tuple<> >(std::piecewise_construct_t const&, std::tuple<void* const&>&&, std::tuple<>&&) ()
#17 0x0000000000401368 in std::_Rb_tree_iterator<std::pair<void* const, int> > std::_Rb_tree<void*, std::pair<void* const, int>, std::_Select1st<std::pair<void* const, int> >, std::less<void*>, std::allocator<std::pair<void* const, int> > >::_M_emplace_hint_unique<std::piecewise_construct_t const&, std::tuple<void* const&>, std::tuple<> >(std::_Rb_tree_const_iterator<std::pair<void* const, int> >, std::piecewise_construct_t const&, std::tuple<void* const&>&&, std::tuple<>&&) ()
#   ... ad inifintum ...

这一次，程序正在通过它的外观设置它的一个流std::cout。并尝试分配内存。看起来memoryManager设置正确或实际初始化正确。无论哪种方式，当它试图分配一些东西时，分配器都会在std::map中创建一个新节点。这需要内存分配。这需要创建一个新节点。。。看到我拿着这个去哪儿了吗？堆栈跟踪像这样重复了很长一段时间。

总而言之，std::map是一个糟糕的内部结构。（事实上，大多数标准容器都会产生类似的问题）。

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