c++ code producing different outputs through Visual Studio c++ and Gcc -

i have following c++ program:

//============================================================================ // name        : // author      : bryce sandlund // version     : // copyright   : // description : code skeleton //============================================================================  #include <iostream> #include <iomanip> #include <set> #include <vector> #include <algorithm> #include <cmath> #include <complex> #include <cstdlib> #include <sstream> #include <list> #include <map> #include <fstream> #include <string> #include <time.h> #include <queue> #include <tuple> #include <functional> #include <unordered_set> #include <unordered_map>  #define inf 1000000000 #define all(c) (c).begin(),(c).end() #define tr(c,i) for(typeof((c).begin()) = (c).begin(); != (c).end(); ++i) #define ep .00001  using namespace std;  typedef pair<int, int> ii; typedef vector<int> vi; typedef vector<bool> vb; typedef vector<vi> vvi; typedef vector<vb> vvb; typedef vector<ii> vii; typedef vector<double> vd; typedef vector<vd> vvd; typedef long long ll;  vvi adjlist; unordered_map<string, int> targs;  int add(string &s) {     if (targs.count(s))         return targs[s];      targs[s] = targs.size();     adjlist.push_back(vi());     return targs.size()-1; }  void connect(int si, int ti) {     if (si == ti)         return;      (int = 0; < adjlist[si].size(); ++i)     {         if (adjlist[si][i] == ti)             return;     }      adjlist[si].push_back(ti); }  vi bfs(int s) {     queue<ii> q;     q.push(ii(s, -1));      vi dist(adjlist.size(), inf);      while (!q.empty())     {         int top = q.front().first;         int hops = q.front().second;         q.pop();         if (dist[top] != inf)             continue;          dist[top] = hops;         (int = 0; < adjlist[top].size(); ++i)         {             q.push(ii(adjlist[top][i], hops+1));         }     }      return dist; }  int main() {     int casenum = 1;     cout << "case " << casenum << ":" << endl;     string line;     while (getline(cin, line))     {         stringstream ss(line);          string command;         ss >> command;         if (command == "add")         {             string s, t;             ss >> s;              int si = add(s);             if (ss >> t)             {                 int ti = add(t);                  connect(si, ti);                 connect(ti, si);             }         }         else if (command == "connections")         {             string s;             ss >> s;              if (!targs.count(s))             {                 cout << "target not exist" << endl;                 continue;             }              int st = targs[s];             if (adjlist[st].empty())             {                 cout << "no connections" << endl;             }             else             {                 vi dist = bfs(st);                  vi away(adjlist.size(), 0);                 int maxd = -1;                 (int = 0; < dist.size(); ++i)                 {                     if (dist[i] == inf || dist[i] == -1)                         continue;                      ++away[dist[i]];                     maxd = max(maxd, dist[i]);                 }                  (int = 0; <= maxd; ++i)                 {                     cout << << ": " << away[i] << endl;                 }             }         }         else if (command == "associated")         {             string s, t;             ss >> s >> t;              if (!targs.count(s) || !targs.count(t))             {                 cout << "target not exist" << endl;                 continue;             }              int si = targs[s], ti = targs[t];             vi dist = bfs(si);              if (dist[ti] == inf)             {                 cout << "no" << endl;             }             else             {                 cout << "yes " << dist[ti] << endl;             }         }         else         {             adjlist.clear();             targs.clear();             cout << "----------" << endl;             cout << "case " << ++casenum << ":" << endl;         }     }      cout << "----------" << endl;     return 0; } 

i using input:

add b add c add b d add e b add c f add c g add f h add h add j k associated associated associated f k associated h connections connections add k g associated j connections add h connections associated h add m add n n connections n add n connections n 

in visual c++, code produces output (it on debug or release):

case 1: yes: 3 yes: 3 no yes: 2 0: 2 1: 4 2: 1 3: 1 0: 1 1: 1 2: 1 3: 2 4: 1 5: 2 yes: 3 0: 2 1: 4 2: 2 3: 2 0: 3 1: 5 2: 1 3: 1 yes: 0 no connections 0: 1 1: 3 2: 5 3: 1 4: 1 ---------- 

on gcc g++, produces output:

case 1: no no yes 0 no 0: 2 1: 2 2: 2 3: 1 0: 1 no 0: 2 1: 2 2: 2 3: 1 0: 3 1: 2 2: 2 3: 1 yes 0 ---------- 

for reference, trying solve problem: https://icpcarchive.ecs.baylor.edu/index.php?option=com_onlinejudge&itemid=8&category=620&page=show_problem&problem=4574.

any ideas why input , output different on different compilers? don't believe using undefined behavior.

the cause of differences in behavior of code in 2 platforms add function. have:

int add(string &s) {     if (targs.count(s))         return targs[s];      targs[s] = targs.size();     adjlist.push_back(vi());     return targs.size()-1; } 

in function, problematic line is:

    targs[s] = targs.size(); 

the line tricky because depending on side of assignment operator gets evaluated first, different behavior. please note targs[s] involves function call changes object.

you can change function little bit make consistent , predictable across platforms.

int add(string &s) {     if (targs.count(s))         return targs[s];      int size = targs.size();     targs[s] = size;     adjlist.push_back(vi());     return size; }