for循環(huán)中刪除map中的元素valgrind檢測提示error：Invalid read of size 8

來源：本站原創(chuàng)|時間：2020-01-10|欄目：C語言|點擊：次

廢話不多說，先看下面一段代碼

#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(xiàn)功能要求，但是健壯性并不好，假設(shè)在map.erase之后再次使用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(xiàn)錯誤，因為it目前指向的對象已經(jīng)被刪掉了。

為了避免程序出現(xiàn)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(jīng)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);
}
}

以上所述是小編給大家介紹的for循環(huán)中刪除map中的元素valgrind檢測提示error：Invalid read of size 8 ，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對我們網(wǎng)站的支持！

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