C++ crashing:push_back 嵌套向量

Question

#include <iostream>
#include <vector>

using namespace std;

struct Neighbor
{
    int index;
    int weight;
    Neighbor(int, int);
};

Neighbor::Neighbor(int index, int weight)
{
    Neighbor::index = index;
    Neighbor::weight = weight;
}

void addEdge(vector<vector<Neighbor> > &graph, int v1, int v2, int weight)
{
    graph[v1].push_back(Neighbor(v2, weight));
    graph[v2].push_back(Neighbor(v1, weight));
}

int main()
{
    vector<vector<Neighbor> > graph;
    vector<vector<Neighbor> > graphIterator;
    graph[0].push_back(Neighbor(1, 5));
    graph[0].push_back(Neighbor(3, 3));
    graph[0].push_back(Neighbor(4, 2.5));

    graph[1].push_back(Neighbor(0, 5));
    graph[1].push_back(Neighbor(2, 3));

    graph[2].push_back(Neighbor(1, 3));
    graph[2].push_back(Neighbor(4, 2.5));
    graph[2].push_back(Neighbor(3, 5));

    graph[3].push_back(Neighbor(0, 3));
    graph[3].push_back(Neighbor(2, 5));
    graph[3].push_back(Neighbor(4, 2.5));

    graph[4].push_back(Neighbor(0, 2.5));
    graph[4].push_back(Neighbor(2, 2.5));
    graph[4].push_back(Neighbor(3, 2.5));

    return 0;
}

以上是我的代码，运行时似乎崩溃了。尽管声明矢量图似乎工作正常，但即使我包含第一个 push_back 语句，程序也会崩溃。有人可以给我建议吗？

Answer 1

当graph[0].push_back(Neighbor(1, 5));时，graph仍然是空的，没有元素，graph[0]导致UB。

你应该先添加元素，比如：

graph.push_back(vector<Neighbor>()); // add one element
...

或

vector<vector<Neighbor> > graph(5); // construct vector with 5 elements

或

vector<vector<Neighbor> > graph;
graph.resize(5);                    // resize vector to contain 5 elements.

Answer 2

graph 创建为空，然后您尝试访问不存在的 graph[0]，... graph[4]。

您可以最初将其声明为 vector<vector<Neighbor> > graph(5);，以便它初始化时包含 5 个 Neighbor 的空向量。

Answer 3

这里

vector<vector<Neighbor> > graph;
/*...*/
graph[0].push_back(Neighbor(1, 5));

您正在访问尚未创建的 graph[0]。如果您创建一个 typedef 到 vector<Neighbor>.

，则效果最好

typedef vector<Neighbor> NeighborVec;
vector<NeighborVec> graph;
NeighborVec& firstVec = graph[0];

可以看清楚，虽然graph已经初始化了，但是graph[0]还没有。你需要做：

typedef vector<Neighbor> NeighborVec;
vector<NeighborVec> graph;
graph.push_back(NeighborVec());
NeighborVec& firstVec = graph[0];
firstVec.push_back(Neighbor(1, 5));

tl;博士:

您忘记初始化嵌套向量的第一层。

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