课堂内外，为什么相同的数据有不同的值？

Question

As the topic, I was confusing this question for two night.

I tried to use function build_BDDs to modify the value of map data structure outs, but the value printed inside the function is not the same as outside the function.

I changed the bool elements of struct map_Data to int type and had the same problem. I suspect I'm using the wrong pointer, but can't figure out what's wrong.

here is my codes.

#include <bits/stdc++.h>
using namespace std;

struct map_Data{
    bool hi;
    bool lo;
};
map<pair<int, int>, map_Data*> outs;

class MWVC{
    public:

    map_Data build_BDDs(map<pair<int, int>, map_Data*> & ins){
        map_Data txt;
        txt.hi = 1;
        txt.lo = 0;
        ins[make_pair(2,4)] = &txt;
        cout << ins[make_pair(2,4)]->hi << ";" << ins[make_pair(2,4)]->lo << endl;
 
        return * ins[make_pair(2,4)];
    }
};


int main(){
    MWVC ojb;
    map_Data a;
    a.hi = 1;
    a.lo = 0;
    outs[make_pair(1,2)] = &a;
    if(outs[make_pair(1,4)]){
        cout << outs[make_pair(1,4)]->hi << endl;
    }

    map_Data b = ojb.build_BDDs(outs);
   
    cout << outs[make_pair(2,4)]->hi << ";" << outs[make_pair(2,4)]->lo << ";" << b.hi << ";" << b.lo << endl;
    return 0;
}

and the result is

1;0
0;0;1;0

Thanks advance.

Answer 1

You build_BDDs function stores the address of a local variable in the map. When the function returns, the memory it points to can be overwritten at any time.

For this instance, just drop the pointers and let the std::map worry about memory management:

map<pair<int, int>, map_Data> outs;

and change build_BDDs to:

map_Data build_BDDs(map<pair<int, int>, map_Data> & ins){
        ins[make_pair(2,4)] = map_Data{1, 0};
        cout << ins[make_pair(2,4)].hi << ";" << ins[make_pair(2,4)].lo << endl;
 
        return ins[make_pair(2,4)];
    }