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

废话不多说，先看下面一段代码

#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(1,"1");
a.mapInsert(2,"2");
a.mapInsert(3,"3");
a.mapInsert(4,"4");
a.mapInsert(5,"5");
a.deleteMap();
return 0;
}

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::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::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::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> >) (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);
}
}

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