Retrieve and store data from sqlite to array c/c++ - c++

Question

I have a column of integers(containing port numbers) in my sqlite data base. Now I want to retrieve list of ports from sqlite in an array form in a function and send data to those port numbers. I tried to make a struct having pointer to Vector but it didn't work out. Making char pointers caused Segmentation error. Here is my code:
struct ConnIDs{
vector<int*> *ids;
};
static int callback(void *NotUsed, int argc, char **argv, char **szColName)
{
ConnIDs *first=(ConnIDs *)NotUsed;
for(int i = 0; i < argc; i++)
{
if (strcmp(szColName[i], "ConnID") == 0)
{
first->ids->push_back(argv[i]);
}
std::cout << szColName[i] << " = " << argv[i] << std::endl;
}
std::cout << "\n";
return 0;
}
int main(){
int result;
sqlite3 *db;
ConnIDs first;
int count = 0;
char *szErrMsg = 0;
int rc = sqlite3_open("Sqlite_Test.db", &db);
if(rc)
{
std::cout << "Can't open database\n";
} else
{
std::cout << "Open database successfully\n";
}
char *pSQL2[1];
pSQL2[0] = "Select * from Subscribers";
rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);
if(rc != SQLITE_OK)
{
std::cout << "SQL Error: " << szErrMsg << std::endl;
sqlite3_free(szErrMsg);
break;
}
else
{
printf("count: %s\n", first.ids);
}
}

Answer 1

This isn't an error regarding sqlite at all, it's just an uninitialised pointer. Let's look at the relevant code:
struct ConnIDs{
vector<int*> *ids;
};
...
ConnIDs first;
rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);
...
static int callback(void *NotUsed, int argc, char **argv, char **szColName)
{
ConnIDs *first=(ConnIDs *)NotUsed;
for(int i = 0; i < argc; i++)
if (strcmp(szColName[i], "ConnID") == 0)
first->ids->push_back(argv[i]);
}
I've omitted unnecessary parts. first is an object of ConnIDs, which is a std::vector<int*>* - it's a pointer to a vector. You pass first into your callback, then immediately try to push into ids; however, you haven't initialised ids, so it's just pointing to rubbish.
The simplest fix is to just not make it a std::vector<int*>* and instead just make it std::vector<int*>, although I'm not certain int* is the correct type here, it will really depend on the data in your database. I would imagine either just int or std::string may be more applicable, but I'll leave that to you:
With the above change, your code will work as-is but you'll need to change first->ids->push_back to first->ids.push_back
Additional notes:
char *pSQL2[1];
pSQL2[0] = "Select * from Subscribers";
rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);
This is kinda weird. I'd probably either just write rc = sqlite3_exec(db, "Select * from Subscribers", ...) or
auto query = "Select * from Subscribers";
rc = sqlite3_exec(db, query.c_str(), callback, &first, &szErrMsg);