Effective STL 学习笔记 32 ~ 33

时间：2021-05-03 15:14:50

Effective STL 学习笔记 32 ~ 33

*/-->

div.org-src-container {
font-size: 85%;
font-family: monospace;
}
pre.src {
background-color:#f8f4d7
}
p {font-size: 15px}
li {font-size: 15px}

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 $

相关文章