for循环中删除map中的元素，valgrind检测提示error：Invalid read of size 8

 #include <iostream>
 #include <map>

 using namespace std;

 class A
 {
     public:
     typedef std::map<int, string> myMap;

     void mapInsert(int i, string s)
     {
         map.insert(std::make_pair(i, s));
     }

     void deleteMap()
     {
         for (myMap::iterator it = map.begin(); it != map.end(); ++it)
         {
             map.erase(it->first);
         }
     }
     private:
     myMap map;
 };

 int main()
 {
     A a;
     a.mapInsert(, "");
     a.mapInsert(, "");
     a.mapInsert(, "");
     a.mapInsert(, "");
     a.mapInsert(, "");

     a.deleteMap();

     return ;
 }

上述代码编译运行皆没有问题，但是用valgrind检测会提示错误：

valgrind --tool=memcheck --leak-check=full --track-origins=yes ./test                                                                                                   # ~/test
==723953== Memcheck, a memory error detector
==723953== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==723953== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==723953== Command: ./test
==723953==
==723953== Invalid read of size 8
==723953==    at 0x3431C69E60: std::_Rb_tree_increment(std::_Rb_tree_node_base*) (tree.cc:60)
==723953==    by 0x40131C: std::_Rb_tree_iterator<std::pair<int const, std::string> >::operator++() (in /home/thm/test/test)
==723953==    by 0x40117C: A::deleteMap() (in /home/thm/test/test)
==723953==    by 0x400F4B: main (in /home/thm/test/test)
==723953==  Address 0x4c580b8 is 24 bytes inside a block of size 48 free'd
==723953==    at 0x4A06016: operator delete(void*) (vg_replace_malloc.c:480)
==723953==    by 0x401E23: __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<int const, std::string> > >::deallocate(std::_Rb_tree_node<std::pair<int const, std::string> >*, unsigned long) (in /home/thm/test/test)
==723953==    by 0x401C99: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::_M_put_node(std::_Rb_tree_node<std::pair<int const, std::string> >*) (in /home/thm/test/test)
==723953==    by 0x401AA6: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::_M_destroy_node(std::_Rb_tree_node<std::pair<int const, std::string> >*) (in /home/thm/test/test)
==723953==    by 0x401729: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::erase(std::_Rb_tree_iterator<std::pair<int const, std::string> >) (in /home/thm/test/test)
==723953==    by 0x40134C: std::map<int, std::string, std::less<int>, std::allocator<std::pair<int const, std::string> > >::erase(std::_Rb_tree_iterator<std::pair<int const, std::string> >) (in /home/thm/test/test)
==723953==    by 0x401170: A::deleteMap() (in /home/thm/test/test)
==723953==    by 0x400F4B: main (in /home/thm/test/test)
==723953==
==723953== Invalid read of size 8
==723953==    at 0x3431C69E80: std::_Rb_tree_increment(std::_Rb_tree_node_base*) (tree.cc:68)
==723953==    by 0x40131C: std::_Rb_tree_iterator<std::pair<int const, std::string> >::operator++() (in /home/thm/test/test)
==723953==    by 0x40117C: A::deleteMap() (in /home/thm/test/test)
==723953==    by 0x400F4B: main (in /home/thm/test/test)
==723953==  Address 0x4c580a8 is 8 bytes inside a block of size 48 free'd
==723953==    at 0x4A06016: operator delete(void*) (vg_replace_malloc.c:480)
==723953==    by 0x401E23: __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<int const, std::string> > >::deallocate(std::_Rb_tree_node<std::pair<int const, std::string> >*, unsigned long) (in /home/thm/test/test)
==723953==    by 0x401C99: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::_M_put_node(std::_Rb_tree_node<std::pair<int const, std::string> >*) (in /home/thm/test/test)
==723953==    by 0x401AA6: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::_M_destroy_node(std::_Rb_tree_node<std::pair<int const, std::string> >*) (in /home/thm/test/test)
==723953==    by 0x401729: std::_Rb_tree<int, std::pair<int const, std::string>, std::_Select1st<std::pair<int const, std::string> >, std::less<int>, std::allocator<std::pair<int const, std::string> > >::erase(std::_Rb_tree_iterator<std::pair<int const, std::string> >) (in /home/thm/test/test)
==723953==    by 0x40134C: std::map<int, std::string, std::less<int>, std::allocator<std::pair<int const, std::string> > >::erase(std::_Rb_tree_iterator<std::pair<int const, std::string> >) (in /home/thm/test/test)
==723953==    by 0x401170: A::deleteMap() (in /home/thm/test/test)
==723953==    by 0x400F4B: main (in /home/thm/test/test)
==723953==
==723953==
==723953== HEAP SUMMARY:
==723953==     in use at exit: 0 bytes in 0 blocks
==723953==   total heap usage: 10 allocs, 10 frees, 370 bytes allocated
==723953==
==723953== All heap blocks were freed -- no leaks are possible
==723953==
==723953== For counts of detected and suppressed errors, rerun with: -v
==723953== ERROR SUMMARY: 8 errors from 2 contexts (suppressed: 6 from 6)

Why?

此代码可以实现功能要求，但是健壮性并不好，假设在map.erase之后再次使用map当前的iterator，即

    void deleteMap()
    {
        for (myMap::iterator it = map.begin(); it != map.end(); ++it)
        {
            map.erase(it->first);
            std::cout << "map.first=" << it->first << "    map.second=" << it->second << std::endl;
        }
    }

代码运行就会出现错误，因为it目前指向的对象已经被删掉了。

为了避免程序出现这样的错误，我们应该保证在iterator指向的对象被删掉之前，iterator已经向前移位一。

程序改成如下即可：

    void deleteMap()
    {
        for (myMap::iterator it = map.begin(); it != map.end();)
        {
            map.erase(it++->first);
        }
    }

或

    void deleteMap()
    {
        for (myMap::iterator it = map.begin(); it != map.end();)
        {
            int i = it->first;
            ++it;
            map.erase(i);
        }
    }