Inside STL: The
shared_ptr constructor vs
There are two ways to create a new object that is controlled by a
// From a raw pointer auto p = std::shared_ptr<S>(new S()); // Via make_shared auto p = std::make_shared<S>();
They result in two different memory layouts.
In the first case, you manually created a new
S object, and then passed a pointer to it to the
shared_ptr constructor. The
shared_ptr adopts the raw pointer and creates a control block to monitor its lifetime. When the last shared pointer destructs, the
Dispose() method deletes the pointer you passed in.¹ When the last shared or weak pointer destructs, the
Delete() method deletes the control block.
In the second case, you let the
make_ function create the
S object, and in practice, what it does is create a single memory allocation that consists of a control block stacked on top of an
S object. This time, when the last shared pointer destructs, the
Dispose() method runs the
S destructor, but the memory isn’t freed yet. Only when the last shared or weak pointer destructs does the
Delete() method get called to free the entire memory block.
|object||•||control_block||freed as a unit|
The two memory layouts have their own pros and cons.
|Two allocations||Single allocation|
Object memory freed
Object memory not freed
|Last shared or weak
|Control block destructs
Control block freed
|Control block destructs
Combo block freed
|Straggler weak pointer||Control block lingers
(Object memory freed already)
|Entire combo block lingers|
The single-allocation version has better memory locality since the control block is kept right next to the managed object.
On the other hand, with the single-allocation version, a straggler weak pointer (a weak pointer which lives for a long time after the last shared pointer has destructed) prevents the entire combo memory block from being freed. By comparison, only the control block remains in memory with the two-allocation version.
If your weak pointers exist to break circular references, then you won’t have stragglers because they will go away when the object graph destructs. Similarly, if your weak pointers are in event handlers, then those weak pointers won’t be stragglers if you are careful to unregister the event handlers at destruction. The stragglers come into play if you retain weak references in, say, a cache or other long-lifetime storage, and even then, they cause a problem only if
sizeof(S) is significant or if you have a lot of them.
Next time, we’ll look at
make_shared‘s close friend,
¹ More specifically, the
Deleter object deletes the pointer you passed in. The default deleter uses the
delete operator to delete the pointer.