关于C++中对于STL容器的一点探究

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原文

http://www.lucien.ink/archives/309/

起因

  很久以前遇到过一个Bug,就是在for循环中使用iterator遍历容器时erase这个指针,导致了一些意想不到的Bug,今天突然想起来,做了点探究。

过程

std::set

  对于set,执行以下代码:

#include <bits/stdc++.h>
std::set<int> set;
void show(std::set<int> &tmp) {
    for (int i : tmp) std::cout << i << ' ';
    std::cout << std::endl;
}
int main() {
    for (int i = 1; i <= 5; i++) set.insert(i);
    for (auto it = set.begin(); it != set.end(); it++) {
        std::cout << "Delete: " << *it << std::endl;
        set.erase(it);
        std::cout << "Current iterator: " <<  *it << std::endl << "Content: ";
        show(set);
        std::cout << std::endl;
    }
    return 0;
}

  输出为:

Delete: 1
Current iterator: 1
Content: 2 3 4 5 

Delete: 2
Current iterator: 2
Content: 3 4 5 

Delete: 3
Current iterator: 3
Content: 4 5 

Delete: 4
Current iterator: 4
Content: 5 

Delete: 5
Current iterator: 5
Content: 

Process finished with exit code 0

std::vector

  对于vector,执行以下代码:

#include <bits/stdc++.h>
std::vector<int> vector;
void show(std::vector<int> &tmp) {
    for (int i : tmp) std::cout << i << ' ';
    std::cout << std::endl;
}
int main() {
    for (int i = 1; i <= 5; i++) vector.push_back(i);
    for (auto it = vector.begin(); it != vector.end(); it++) {
        std::cout << "Delete: " << *it << std::endl;
        vector.erase(it);
        std::cout << "Current iterator: " <<  *it << std::endl << "Content: ";
        show(vector);
        std::cout << std::endl;
    }
    return 0;
}

  输出为:

Delete: 1
Current iterator: 2
Content: 2 3 4 5 

Delete: 3
Current iterator: 4
Content: 2 4 5 

Delete: 5
Current iterator: 5
Content: 2 4 

Delete: 5

Process finished with exit code 11

结论

  可以观察到,erasestd::set的指针后这个指针不变,且指向的内容仍然有效,并且能正确的获得中序遍历的后继。

  erasestd::vector的指针后这个指针会自动指向被erase掉的内容的下一个元素,并且由于容器中的元素为奇数个,上文的代码exit code为$11$,可以预见的是,如果容器中的元素为偶数个,那么我们的这一段代码就不会出错,即exit code为$0$。

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