for循環中刪除map中的元素，valgrind檢測提示error：Invalid read of size 8，mapvalgrind
1 #include <iostream>
2 #include <map>
3
4 using namespace std;
5
6 class A
7 {
8 public:
9 typedef std::map<int, string> myMap;
10
11 void mapInsert(int i, string s)
12 {
13 map.insert(std::make_pair(i, s));
14 }
15
16 void deleteMap()
17 {
18 for (myMap::iterator it = map.begin(); it != map.end(); ++it)
19 {
20 map.erase(it->first);
21 }
22 }
23 private:
24 myMap map;
25 };
26
27 int main()
28 {
29 A a;
30 a.mapInsert(1, "1");
31 a.mapInsert(2, "2");
32 a.mapInsert(3, "3");
33 a.mapInsert(4, "4");
34 a.mapInsert(5, "5");
35
36 a.deleteMap();
37
38 return 0;
39 }
上述代碼編譯運行皆沒有問題，但是用valgrind檢測會提示錯誤：
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?
此代碼可以實現功能要求，但是健壯性並不好，假設在map.erase之後再次使用map當前的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;
}
}
代碼運行就會出現錯誤，因為it目前指向的對象已經被刪掉了。
為了避免程序出現這樣的錯誤，我們應該保證在iterator指向的對象被刪掉之前，iterator已經向前移位一。
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);
}
}
