#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);
}
}