Question

graph.h

struct Edge {
    int from;
    int to;
    unsigned int id;
    Edge(): from(0), to(0), id(0) {};
};

struct Vertex {
    int label;
    vector<Edge> edge;
};

class Graph: public vector<Vertex> {
    int gid;
    unsigned int edge_size;
};

class Trans {
public: 
    int tid;
    vector<Graph> graph;
};

vector<Trans> database; database is a global variable, then i call run_algo(database); in main function.

void run_algo(vector<Trans> &db) {
    EdgeList edges;
    for(unsigned int tid = 0; tid < db.size(); tid++) {
            Trans &t = db[tid];
            ...
       Graph g = t.graph[gid];    

I want to ask db is a alias to database, db[tid] is a Transaction vector, but what if the difference between using Trans &t = db[tid]; and Trans t = db[tid];, since the author who write the sample using Trans &t = db[tid];, but i think it should use Trans t = db[tid];

Thanks:)

Answer 1

After

Trans &t = db[tid];

t is and behaves exactly as the item in db[tid], you change t, you change db[tid]

With

Trans t = db[tid];

t is merely a copy of the item in db[tid], changing t won’t change db[tid] here.