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Table of Contents
记得 Remove 后要 Erase
remove, container, pointer
1 记得 Remove 后要 Erase
Item 9 中已经提到过，如果真的想要用 remove 从容器中删除元素，必须要在 remove 之后使用 erase
。其原因在于： remove 接受的参数是通用的迭代器，而不是容器，它不知道容器的任何信息，也就无法从容器中删除元素： remove doesn't really remove anything, because it can't.
remove 只保证在其返回的迭代器之前的元素为有效数据（符合条件的数据），但它既无法删除被 remove
的数据，也不能保证返回的迭代器之后的数据位无效数据（不符合条件的数据）。这一点与某些容器内置的 remove member function 不一样：这些 member function 会真正的把数据删除掉：
// For vector, we have to call erase after remove.
vector<int> v;
// ....
vector<int>::iterator endPos = remove(v.begin(), v.end(), 99); // Remove 99 from vector, returns last valid position.
v.erase(posEnd, v.end()); // Erase all elements after posEnd. // For list, the member function remove is enough:
list<int> l;
//...
l.remove(99); // this will erase all 99 in the list.
2 remove, container, pointer
Item 6 中提到，在容器中放置对象指针容易混乱，这里如果对存放指针的容器调用 remove-erase idom
的话，会发生什么事情？
1: class Widget
2: {
3: public:
4: Widget();
5: virtual ~Widget();
6: bool IsCertified();
7: };
8:
9: vector<Widget*> v;
10:
11: // push lots of widget pointers to v.
12:
13: v.erase(remove(v.begin(), v.end(), not1(mem_fun(&Widget::IsCertified))), v.end());
内存泄露。
那么将第 13 行换成下面的表达式呢？
14: vector<Widget*> endPos = remove(v.begin(), v.end(), not(mem_fun(&Widget::IsCertified)));
15: for_each(endPos, v.end(), [](Widget* pw){if(pw) delete pw;});
16: v.erase(endPos, v.end());
第 14 行将所有的 endPos 之后的指针先释放，然后再去 erase。前面刚刚提过， remove 不会保证返回的迭代器之后的元素都为无效值，第 14 行有可能会将本该保留的对象给释放掉，还有可能会将该释放的不释放。结果可能会 Crash 或者内存泄露。
我们应该保证在 remove 过程中，一旦发现不合要求的数据，马上将其删除，例如下面的例子：
#include <vector>
#include <iterator>
#include <algorithm>
#include <iostream>
#include <stdio.h> using namespace std; class Widget
{
public:
Widget(int i);
virtual ~Widget();
bool IsCertified();
void Show();
private:
int m_i;
}; /* See description in header file. */
Widget::Widget(int i)
: m_i(i)
{
} /* See description in header file. */
Widget::~Widget()
{
printf ("Deleting Obj: %p, value: %d\n", this, m_i);
} /* See description in header file. */
bool Widget::IsCertified()
{
return m_i % 2 == 0;
} /* See description in header file. */
void Widget::Show()
{
printf ("\tObj: %p, value: %d\n", this, m_i);
} bool DeleteIfUncitified(Widget* p)
{
if (p && !p->IsCertified())
{
delete p;
return true;
}
return false;
} int main(int argc, char *argv[])
{
vector<Widget*> v;
for (int i = 0; i < 20; ++i)
{
Widget* p = new Widget(i);
v.push_back(p);
} random_shuffle(v.begin(), v.end()); printf ("Before remove, size: %lu, contents:\n", v.size());
for_each(v.begin(), v.end(),
[](Widget* p){
if (p) p->Show();
else printf("Obj is Null");});
printf ("\nNow removing...\n");
v.erase(remove_if(v.begin(), v.end(), DeleteIfUncitified), v.end());
printf ("\nAfter remove, size: %lu, contents:\n", v.size());
for_each(v.begin(), v.end(),
[](Widget* p){
if (p) p->Show();
else printf("Obj is Null");}); return 0;
}
其输出为：
Welcome to the Emacs shell ~/tmp $ ./test
Before remove, size: 20, contents:
Obj: 0x7f8b31c038d0, value: 19
Obj: 0x7f8b31c03870, value: 3
Obj: 0x7f8b31c039e0, value: 14
Obj: 0x7f8b31c039f0, value: 15
Obj: 0x7f8b31c03890, value: 10
Obj: 0x7f8b31c03850, value: 2
Obj: 0x7f8b31c03900, value: 6
Obj: 0x7f8b31c038b0, value: 17
Obj: 0x7f8b31c03920, value: 8
Obj: 0x7f8b31c038a0, value: 4
Obj: 0x7f8b31c039c0, value: 12
Obj: 0x7f8b31c03910, value: 7
Obj: 0x7f8b31c038f0, value: 5
Obj: 0x7f8b31c039b0, value: 11
Obj: 0x7f8b31c03860, value: 1
Obj: 0x7f8b31c03880, value: 9
Obj: 0x7f8b31c03840, value: 0
Obj: 0x7f8b31c03a00, value: 16
Obj: 0x7f8b31c039d0, value: 13
Obj: 0x7f8b31c038c0, value: 18 Now removing...
Deleting Obj: 0x7f8b31c038d0, value: 19
Deleting Obj: 0x7f8b31c03870, value: 3
Deleting Obj: 0x7f8b31c039f0, value: 15
Deleting Obj: 0x7f8b31c038b0, value: 17
Deleting Obj: 0x7f8b31c03910, value: 7
Deleting Obj: 0x7f8b31c038f0, value: 5
Deleting Obj: 0x7f8b31c039b0, value: 11
Deleting Obj: 0x7f8b31c03860, value: 1
Deleting Obj: 0x7f8b31c03880, value: 9
Deleting Obj: 0x7f8b31c039d0, value: 13 After remove, size: 10, contents:
Obj: 0x7f8b31c039e0, value: 14
Obj: 0x7f8b31c03890, value: 10
Obj: 0x7f8b31c03850, value: 2
Obj: 0x7f8b31c03900, value: 6
Obj: 0x7f8b31c03920, value: 8
Obj: 0x7f8b31c038a0, value: 4
Obj: 0x7f8b31c039c0, value: 12
Obj: 0x7f8b31c03840, value: 0
Obj: 0x7f8b31c03a00, value: 16
Obj: 0x7f8b31c038c0, value: 18
~/tmp $
（转载请注明出处，使用许可：署名-非商业性使用-相同方式共享 3.0 中国大陆许可协议 。）
