How to use shared_pointer as value in a std::map structure? - c++

Question

I have a map structure as below which has shared_pointer of a protobuf structure Stats:
map<string, shared_pointer<vol::proto::Stats> statsMap;
I am storing and retrieving the map entries through a LRU cache implementation
template class LRUCache<string,std::shared_ptr<vol::proto::Stats>>;
Protobuf structure:-
message Stats {
required string oid = 1;
required int64 logical_size = 2;
required int64 committed_size = 3;
}
message StatsRequest {
required string uuid = 1;
}
The function for storing entry into the map:-
template<class K, class V>
void
LRUCache<K, V>::Put(const K& key, V value)
{
Vmacore::System::WriteSynchronized lock(this);
auto val = refMap.find(key);
if (val == refMap.end()) {
if (cacheSize == refMap.size()) {
K leastUsedKey = lruList.back();
lruList.pop_back();
refMap.erase(leastUsedKey);
}
} else {
lruList.erase(val->second.lruKeyRef);
}
lruList.push_front(key);
refMap[key] = LRUValueReference<K, V>(lruList.begin(), value);
}
void PutS(const string& Id, const vol::proto::Stats& info)
{
shared_ptr<vol::proto::Stats> statsInfo = make_shared<vol::proto::Stats>(info);
_StatsCache.Put(Id, statsInfo);
}
void PutStats(vol::proto::StatsRequest &req) {
vol::proto::Stats *stats;
GetStats(stats); //stats gets populated here.
PutS(stats->oid(), *stats);
}
To Get the entry from the map:-
template<class K, class V>
bool
LRUCache<K, V>::Get(const K& key, V& value)
{
Vmacore::System::WriteSynchronized lock(this);
auto val = refMap.find(key);
if (val == refMap.end()) {
return false;
}
lruList.splice(lruList.begin(), lruList, val->second.lruKeyRef);
val->second.lruKeyRef = lruList.begin();
value = val->second.value;
return true;
}
void GetS(const string& Id, shared_ptr<vol::proto::Stats> info)
{
if (!_StatsCache.Get(Id, info))
return false;
return true;
}
void GetStats(vol::proto::StatsRequest &req) {
shared_ptr<vol::proto::Stats> stats;
if (GetS(req.uuid(), stats)) {
Log("object id is %s", stats.get()->oid()); // Crash at oid (this=0x0)
}
}
Now while doing GetStats, I am seeing a crash as to in GetStats's Log statement that is program terminated with signal SIGSEGV, Segmentation fault. Should I do a make_share for Stats in GetStats? Can you please describe as to what has caused this crash and how to fix the crash here?

Answer 1

You're passing a raw pointer from GetStats to GetS, which then wraps it into a shared_ptr. Now lets start ref-counting.
GetS -> increase on entry > 1
_StatsCache.Get -> pass by ref > 1
GetS -> decrease on exit > 0
Ref count 0 -> delete the pointer.
In GetStats you then try to de-reference the stats pointer by calling .get, but it was just deleted.
You should either always use smart pointers, or be very very very careful when you convert from raw pointer to smart pointer, and back.
Actually what I wrote is wrong. What you're doing is actually much worse.
You're wrapping a raw pointer in GetS into a smart pointer, and then pass it by ref to your _StatsCache.Get, which then assigns a different shared_ptr to yours. So you've overwritten the original raw pointer with something else.
But when GetS returns, that shared_ptr is now lost and destructed, but the original stats pointer is accessed in stats.get(). What's in it? Nothing from GetS, that's for sure.