实战c++中的vector系列--再谈vector的insert()方法(都是make_move_iterator惹的祸)

时间:2023-03-08 22:00:13
实战c++中的vector系列--再谈vector的insert()方法(都是make_move_iterator惹的祸)

之前说过了关于vector的insert()方法,把vector B的元素插入到vector A中。vector A中的结果我们可想而知,可是vector B中的元素还会怎样?

看看之前写过的程序:

#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it = myvector.insert ( it , 200 ); myvector.insert (it,2,300); // "it" no longer valid, get a new one: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;
}

如今关心一下别的:注意是insert后,被insert的vector为多少了:

#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;
}
//输出:
//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

假设你看到此时,你肯定会在心里骂娘,谁还关心vector B,而且vectorB并没有变化。

如今是时候来点猛药了。vector中放智能指针。

之前博客也讲诉过对于vector的元素为智能指针的时候:

#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;
}
}

上面代码会崩溃。原因就是vec被insert后 。vec变得无效了,我们不能对他做什么。。。。。

可是须要明白的是这不是insert造成的,假设copy也会造成这一的结局,事实上罪魁祸首就是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';
}
//输出:
//v1 now holds : "one" "two" "three"
//v2 now holds : "one" "two" "three"
//original list now holds : ""

最后再上一个官方程序:

#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;
}

* 须要注意:*

* bar.clear();*

因此此时: bar now contains unspecified values; clear it: