實戰c++中的vector系列--再談vector的insert()方法(都是make_move_iterator惹的禍)

之前說過了關於vector的insert()方法，把vector B的元素插入到vector A中。vector A中的結果我們可想而知，可是vector B中的元素還會怎樣？

看看之前寫過的程序：

#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector (3,100);
  std::vector<int>::iterator it;

  it = myvector.begin();
  it = myvector.insert ( it , 200 );

  myvector.insert (it,2,300);

  // "it" no longer valid, get a new one:
  it = myvector.begin();

  std::vector<int> anothervector (2,400);
  myvector.insert (it+2,anothervector.begin(),anothervector.end());

  int myarray [] = { 501,502,503 };
  myvector.insert (myvector.begin(), myarray, myarray+3);

  std::cout << "myvector contains:";
  for (it=myvector.begin(); it<myvector.end(); it++)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

如今關心一下別的：注意是insert后，被insert的vector為多少了：

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> myvector(3, 100);

    std::vector<int> anothervector(2, 400);

    std::cout << "Before insert myvector is:";
    for (auto it = myvector.begin(); it<myvector.end(); it++)
        std::cout << ' ' << *it;
    std::cout << std::endl;

    std::cout << "Before insert anothervector is:";
    for (auto it = anothervector.begin(); it<anothervector.end(); it++)
        std::cout << ' ' << *it;

    std::cout << std::endl;

    myvector.insert(myvector.end(), anothervector.begin(), anothervector.end());

    std::cout << "After insert myvector is:";
    for (auto it = myvector.begin(); it<myvector.end(); it++)
        std::cout << ' ' << *it;

    std::cout << std::endl;

    std::cout << "Now the anothervector is:";
    for (auto it = anothervector.begin(); it<anothervector.end(); it++)
        std::cout << ' ' << *it;

    std::cout << std::endl;

    int myarray[] = { 501,502,503 };
    myvector.insert(myvector.begin(), myarray, myarray + 3);

    std::cout << "After insert myarray[] to myvector, myvector is:";
    for (auto it = myvector.begin(); it<myvector.end(); it++)
        std::cout << ' ' << *it;

    std::cout << std::endl;

    std::cout << "After insert myarray[] to myvector, myarray[] is:";
    for (int i = 0; i < 3; i++)
    {
        std::cout << ' ' << myarray[i];
    }

    return 0;
}
//輸出：
//Before insert myvector is : 100 100 100
//Before insert anothervector is : 400 400
//After insert myvector is : 100 100 100 400 400
//Now the anothervector is : 400 400
//After insert myarray[] to myvector, myvector is : 501 502 503 100 100 100 400 400
//After insert myarray[] to myvector, myarray[] is : 501 502 503

假設你看到此時，你肯定會在心里罵娘，誰還關心vector B，而且vectorB並沒有變化。

如今是時候來點猛葯了。vector中放智能指針。

之前博客也講訴過對於vector的元素為智能指針的時候：

#include<iostream>
#include<vector>
#include <memory>
using namespace std;
void display_vector(vector<unique_ptr<int>> &vec);
int main()
{
    vector<unique_ptr<int>> vec;
    unique_ptr<int> s1(new int(1));
    unique_ptr<int> s2(new int(2));
    unique_ptr<int> s3(new int(3));
    unique_ptr<int> s4(new int(4));
    vec.push_back(std::move(s1));
    vec.push_back(std::move(s2));
    vec.push_back(std::move(s3));
    vec.push_back(std::move(s4));


    unique_ptr<int> s5(new int(5));
    vector<unique_ptr<int>> des_vec;
    des_vec.push_back(std::move(s5));
    des_vec.insert(des_vec.end(), std::make_move_iterator(vec.begin()), std::make_move_iterator(vec.end()));
    display_vector(des_vec);
    cout << "now, des_vec size: " << des_vec.size() << endl;
    cout << "now, vec size: " << vec.size() << endl;

    //display_vector(vec);
    cout << "now, vec size: " << vec.size() << endl;
    for (int i=0; i<vec.size(); i++)
    {
        cout << *vec[i] << " ";
    }
    return 0;
}

void display_vector(vector<unique_ptr<int>> &vec)
{
    for (auto it = vec.begin(); it != vec.end(); it++)
    {
        cout << **it << endl;
    }
}

上面代碼會崩潰。原因就是vec被insert后 。vec變得無效了，我們不能對他做什么。。。。。

可是須要明白的是這不是insert造成的，假設copy也會造成這一的結局，事實上罪魁禍首就是make_move_iterator

再看程序：

#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <iterator>

int main()
{
    std::list<std::string> s{ "one", "two", "three" };

    std::vector<std::string> v1(s.begin(), s.end()); // copy

    std::vector<std::string> v2(std::make_move_iterator(s.begin()),
        std::make_move_iterator(s.end())); // move

    std::cout << "v1 now holds: ";
    for (auto str : v1)
        std::cout << "\"" << str << "\" ";
    std::cout << "\nv2 now holds: ";
    for (auto str : v2)
        std::cout << "\"" << str << "\" ";
    std::cout << "\noriginal list now holds: ";
    for (auto str : s)
        std::cout << "\"" << str << "\" ";
    std::cout << '\n';
}
//輸出：
//v1 now holds : "one" "two" "three"
//v2 now holds : "one" "two" "three"
//original list now holds : ""

最后再上一個官方程序：

#include <iostream> // std::cout
#include <iterator> // std::make_move_iterator
#include <vector> // std::vector
#include <string> // std::string
#include <algorithm> // std::copy

int main() {
    std::vector<std::string> foo(3);
    std::vector<std::string> bar{ "one","two","three" };

    std::copy(make_move_iterator(bar.begin()),
        make_move_iterator(bar.end()),
        foo.begin());

    // bar now contains unspecified values; clear it:
    bar.clear();

    std::cout << "foo:";
    for (std::string& x : foo) std::cout << ' ' << x;
    std::cout << '\n';

    return 0;
}

* 須要注意：*
* bar.clear();*
因此此時: bar now contains unspecified values; clear it: