C++崩溃：嵌套向量上的push_back

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; 
} 

上面是我的代码，运行时似乎崩溃了。尽管声明向量图似乎工作正常，但即使包含我的第一个push_back语句，程序也会崩溃。有人可以提醒我吗？

Answer 1

当graph[0].push_back(Neighbor(1, 5));,仍为空时，它没有元素，并且graph[0]通向UB。

您应该添加的元素首先，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

创建空的，那么您尝试访问graph[0]，... graph[4]这不在那里。

您最初可以声明它为vector<vector<Neighbor> > graph(5);，以便它初始化为包含5个空向量Neighbor s。

Answer 3

这里

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

您正在访问到graph[0]至极尚未创建。如果您创建一个typedef至vector<Neighbor>，这是最好的可视化。

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

你可以清楚地看到，虽然已初始化graph[0]没有。你需要做的：

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

TL;博士：

你忘了初始化向量嵌套的第一级。