is it ok to access value(entry in thread safe map) pointed by pointer inside non-thread safe container?

Question

For example,

// I am using thread safe map from
// code.google.com/p/thread-safe-stl-containers
#include <thread_safe_map.h>

class B{
  vector<int> b1;
};

//Thread safe map
thread_safe::map<int, B> A;
B b_object;
A[1] = b_object;

// Non thread safe map.
map<int, B*> C;
C[1] = &A[1].second;

So are following operations still thread safe?

Thread1:
for(int i=0; i<10000; i++) {
  cout << C[1]->b1[i];
}

Thread2:
for(int i=0; i<10000; i++) {
  C[1]->b1.push_back(i);
}

Is there any problem in the above code? If so how can I fix it?

Is it OK to access value(entry in thread safe map) pointed by pointer inside non-thread safe container?

Answer 1

No, what you are doing there is not safe. The way your thread_safe_map is implemented is to take a lock for the duration of every function call:

//Element Access
T & operator[]( const Key & x ) { boost::lock_guard<boost::mutex> lock( mutex ); return storage[x]; }

The lock is released as soon as the access function ends which means that any modification you make through the returned reference has no protection.

As well as being not entirely safe this method is very slow.

A safe(er), efficient, but highly experimental way to lock containers is proposed here: https://github.com/isocpp/CppCoreGuidelines/issues/924 with source code here https://github.com/galik/GSL/blob/lockable-objects/include/gsl/gsl_lockable (shameless self promotion disclaimer).