C++ thread lambda captured object can read but cannot erase - c++

Question

I'm new in c++ so my problem could be very simple but I cannot solve it.
In my constructor I want to start detached thread which will be looping with class variable and removing old data.
userCache.hpp
struct UserCacheItem {
long m_expires;
oatpp::Object<User> m_user;
UserCacheItem(const long expires, const oatpp::Object<User> &user);
};
class UserCache {
private:
std::map<std::string, std::shared_ptr<UserCacheItem> > m_userCacheItem;
public:
UserCache();
void cacheUser(std::string payload, std::shared_ptr<UserCacheItem> &userCacheItem);
};
userCache.cpp
UserCache::UserCache()
{
std::thread thread([this]() mutable {
while (true){
auto curTimestamp = std::chrono::seconds(std::chrono::seconds(std::time(nullptr))).count();
for(auto &elem : m_userCacheItem){
if (curTimestamp > elem.second->m_expires){
std::cout << "Erasing element: " << elem.second->m_expires << std::endl;
m_userCacheItem.clear();
}
}
std::cout << "Cache size: " << m_userCacheItem.size() << " Current timestamp: " << curTimestamp << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(10));
};
});
thread.detach();
}
When I reach line m_userCacheItem.clear(); I get segmentation fault. Ofcourse if if block is false line with cout cache sizie is printed properly.
So I can read my variable but I cannot modify it :(
Where I'm making error?

Answer 1

You cannot modify the map while you're iterating it
for(auto &elem : m_userCacheItem){
if (curTimestamp > elem.second->m_expires){
std::cout << "Erasing element: " << elem.second->m_expires << std::endl;
m_userCacheItem.clear();
}
}
If you want to erase an element, use std::map::erase:
for(auto & it = m_userCacheItem.begin(); it != m_userCacheItem.end();) {
if(condition) {
it = m_userCacheItem.erase(it);
} else {
++it;
}
}