为什么这些向量不相等? [英] Why aren't these vectors equal?

问题描述

我创建了两个向量，另一个用 push_back 填充，另一个用索引填充.我希望这些是平等的，但事实并非如此.有人能解释一下这是为什么吗?

I've created two vectors and filled the other one with push_back and the other one with indices. I would expect these to be equal but ther aren't. Can someone explain me why is this?

#include <vector>
#include <iostream>

using namespace std;

int main() {

    vector<int> v0;
    v0.push_back(0);
    v0.push_back(1);
    v0.push_back(2);

    vector<int> v1;
    v1.reserve(3);
    v1[0] = 0;
    v1[1] = 1;
    v1[2] = 2;

    if (v0 != v1) {
            cout << "why aren't they equal?" << endl;
    }

    return 0;
}

推荐答案

vector<int> v1;
v1.reserve(3);
v1[0] = 0;
v1[1] = 1;
v1[2] = 2;

这可能是一个未定义的行为(虽然不确定它是否依赖于实现).

This is probably an undefined behavior ( although not sure if it's implementation dependent).

您不能使用 operator[] 来填充向量，因为它返回对底层对象的引用，在您的情况下，它只是一堆位.

You cannot use operator[] for filling up the vector as it's returning the reference to the underlying object which in your case is nothing other than bunch of bits.

你应该使用 push_back() 或者只是 resize 你的向量.使用后者:-

You should either use push_back() OR just resize your vector.Using latter:-

vector<int> v1;
v1.resize(3);
v1[0] = 0;
v1[1] = 1;
v1[2] = 2;

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