Why stack flow before execute main() - c++

Question

I wrote code to implement graph's ADT with adjacent matrix. Then I want to build a project and desperate code.
#include<iostream>
#include<iomanip>
#include<cstring>
#include<string>
#include<cstdio>
#include<vector>
#include<queue>
#include<algorithm>
#define INF 0x3f3f3f3f
using namespace std;
const int maxn=1e3+10;
class Graph{
private:
int vertex;
int side;
int is_vis[maxn];
int degree[maxn];
int Edges[maxn][maxn];
public:
Graph(){
vertex=0;
memset(Edges,0x3f,sizeof Edges);
memset(is_vis,0,sizeof is_vis);
memset(degree,0,sizeof degree);
}
void Clear(){
vertex=0;
memset(Edges,0x3f,sizeof Edges);
memset(is_vis,0,sizeof is_vis);
memset(degree,0,sizeof degree);
}
void Clear_Vis(){memset(is_vis,0,sizeof is_vis);}
void PreVisit(int v){cout<<v<<" ";}
void SetMark(int u,int vis){is_vis[u]=vis;}
void Set_vertex(int n){vertex=n;} //顶点数
int Get_vertex(){return vertex;}
int Get_side() {return side;} //边数
void Add_edge(int u,int v,int val){ //加边
if(Edges[u][v]==INF){
side++;
degree[u]++;degree[v]++;
}
Edges[u][v]=Edges[v][u]=val;
}
void Cancle_edge(int u,int v){ //删边
if(Edges[u][v]!=INF){
side--;
degree[u]--;degree[v]--;
}
Edges[u][v]=Edges[v][u]=INF;
}
bool Have_edge(int u,int v){ //判断边是否存在
return Edges[u][v]==INF?0:1;
}
int Get_degree(int u){ //返回顶点u的度数
return degree[u];
}
void The_edge_node(int u){ //与顶点u相邻的点
int flag=1;
for(int i=0;i<vertex;i++){
if(i!=u&&Edges[u][i]!=INF){
cout<<i<<" ";flag=0;
}
}
if(flag)cout<<"No node next to "<<u<<".";
cout<<endl;
}
void DFS(int u){ //深度优先遍历
if(is_vis[u])return;
PreVisit(u);
SetMark(u,1);
for(int i=0;i<vertex;i++){
if(i==u||Edges[u][i]==INF||is_vis[i])continue;
DFS(i);
}
}
void BFS(int u){ //宽度优先遍历
memset(is_vis,0,sizeof is_vis);
queue<int>Q;
Q.push(u);
is_vis[u]=1;
while(!Q.empty()){
int out=Q.front();Q.pop();
cout<<out<<" ";
for(int i=0;i<vertex;i++){
if(i==out||Edges[out][i]==INF||is_vis[i])continue;
is_vis[i]=1;
Q.push(i);
}
}
}
void prt_Gra(){ //打印邻接矩阵
cout<<" ";
for(int i=0;i<vertex;i++){
cout<<" "<<left<<setw(3)<<i;
}
cout<<endl;
for(int i=0;i<vertex;i++){
cout<<" ";
for(int j=0;j<vertex*4;j++)cout<<"-";cout<<"-"<<endl;
cout<<left<<setw(2)<<i;
for(int j=0;j<vertex;j++){
cout<<"|";
if(i==j)cout<<left<<setw(3)<<0;
else if(Edges[i][j]==INF)cout<<left<<setw(3)<<'-';
else cout<<left<<setw(3)<<Edges[i][j];
}
cout<<"|"<<endl;
}
cout<<" ";
for(int j=0;j<vertex*4;j++)cout<<"-";cout<<"-"<<endl;
}
}My_graph;
void Get_menus(){
cout<<"1.新建一个图。"<<endl;
cout<<"2.增加一条边。"<<endl;
cout<<"3.删除一条边。"<<endl;
cout<<"4.判断边是否存在。"<<endl;
cout<<"5.查询顶点的度数。"<<endl;
cout<<"6.输出图的邻接矩阵。"<<endl;
cout<<"7.查询与顶点相邻的点。"<<endl;
cout<<"8.对图进行深度优先遍历。"<<endl;
cout<<"9.对图进行宽度优先遍历。"<<endl;
}
int main(){
My_graph.Clear();
int cmd,n,m,u,from,to,dist;
Get_menus();
while(1){
cout<<"请输入对应操作的序号：";cin>>cmd;cout<<endl;
if(cmd==1){
My_graph.Clear();
cout<<"请输入图的顶点数：";cin>>n;
My_graph.Set_vertex(n);cout<<endl;
cout<<"请输入图的边数：";cin>>m;cout<<endl;
cout<<"请依次输入各边的端点与长度："<<endl;
for(int i=0;i<m;i++){
cin>>from>>to>>dist;
My_graph.Add_edge(from,to,dist);
}
cout<<"图的初始化完成！"<<endl;
}
if(cmd==2){
cout<<"请输入新增边的端点与长度："<<endl;
cin>>from>>to>>dist;
My_graph.Add_edge(from,to,dist);
cout<<"加边完成！"<<endl;
}
if(cmd==3){
cout<<"请输入边的两个端点：";
cin>>from>>to;
My_graph.Cancle_edge(from,to);
cout<<"删边完成！"<<endl;
}
if(cmd==4){
cout<<"请输入边的两个端点：";
cin>>from>>to;
if(My_graph.Have_edge(from,to))cout<<"该边存在！"<<endl;
else cout<<"该边不存在！"<<endl;
}
if(cmd==5){
cout<<"请输入需要查询度数的顶点：";
cin>>u;cout<<endl;
cout<<"顶点"<<u<<"的度数为："<<My_graph.Get_degree(u)<<endl;
}
if(cmd==6){
cout<<"图的邻接矩阵为："<<endl;
My_graph.prt_Gra();
}
if(cmd==7){
cout<<"请输入需要查询相邻点的顶点：";
cin>>u;cout<<endl;
cout<<"与顶点"<<u<<"相邻的点有："<<endl;
My_graph.The_edge_node(u);
}
if(cmd==8){
My_graph.Clear_Vis();
cout<<"请输入起点：";
cin>>u;cout<<endl;
cout<<"深度优先遍历的结果为："<<endl;
My_graph.DFS(u);cout<<endl;
}
if(cmd==9){
cout<<"请输入起点：";
cin>>u;cout<<endl;
cout<<"宽度优先遍历的结果为："<<endl;
My_graph.BFS(u);cout<<endl;
}
}
return 0;
}
/*
6 9
0 1 10
0 3 20
0 5 3
1 2 3
1 3 5
3 4 11
2 4 15
3 5 10
4 5 3
*/
But when I run the code, CodeBlocks says ''multiple definition of My_graph'.
Then I choose to define Graph's object in main() instead of in Graph.h. But this time the exe will exit before execute main().
Process returned -1073741571 (0xC00000FD)
This is main()code
I want to figure out what happened in constructor.

Answer 1

Even if you are not allocating your data on the stack, you won't be able to execute your code. This is the amount of data you want to allocate:
const int maxn=1e3+10;
int is_vis[maxn];
int degree[maxn];
int Edges[maxn][maxn];
I don't think your computer has enough space.
Use vectors, and resize the edges matrix ont he fly (do you need a matrix anyway? Probably not)
std::vector<int> is_vis;
std::vector<int> degree;
std::vector<std::vector<int>> Edges; // Change this to something sensible, perhaps a list of connections?

Answer 2

This is only for one error message "multiple definition...". For the other error see #MatthieuBrucher answer.
class Graph{
....
} My_graph;
This declares an instance global object of type Graph called My_graph.
Once you have this instance you can not declare it again:
Graph My_graph; //ERROR
This re-declaration may come not only in some function ('main' is a function) , but also from including the same .h header several times without include-guards.