Sync_queue class compilation error

Question

Stroustrup has presented a synchronized queue class Sync_queue. (CPL, 4, pp 1232-1235).

The Sync_queue tester gives the following compilation errors:

In file included from main.cpp:9:

./Sync_queue.h:69:47: error: variable 'min_size' cannot be implicitly captured in a lambda with no capture-default specified
                    [this] {return q.size() < min_size;});
                                              ^
./Sync_queue.h:62:23: note: 'min_size' declared here
         unsigned int min_size)
                      ^
./Sync_queue.h:69:21: note: lambda expression begins here
                    [this] {return q.size() < min_size;});
                    ^
1 error generated.

The relevant portion of the Sync_queue class (where the compilation errors occur) is a method containing a lambda, shown below:

/** During a duration d, if the queue size
    falls below a minimum level,
    put a value onto the queue. */
template<typename T>
bool Sync_queue<T>::put(
         T val, 
         std::chrono::steady_clock::duration d,
         unsigned int min_size)
{
   std::unique_lock<std::mutex> lck {mtx};

   bool low = cnd.wait_for(
                    lck,
                    d,
                    [this] {return q.size() < min_size;});

   if (low)
   {
      q.push_back(val);
      cnd.notify_one();
   }
   else
   {
      cnd.notify_all();
   }

   return low;
}

The following line is line 62:

     unsigned int min_size)

and line 69 contains the lambda as a predicate to the condition_variable's wait_for() method:

                [this] {return q.size() < min_size;});

How do I fix this compilation error?

Answer 1

To use min_size inside a lambda expression you should explicitly capture it:

[this, min_size] {return q.size() < min_size;}

Another option would be to use automatic captures like [&] or [=] but in general it's better to use explicit captures to prevent your lambda from unexpected side effects. For detailed explanation see cppreference on lambdas - captures item.