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POAdding an allocator to a C++ class template for shared memory object creation
 

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  1. POAdding an allocator to a C++ class template for shared memory object creation
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    <p>In short, my question is: If you have class, <code>MyClass&lt;T&gt;</code>, how can you change the class definition to support cases where you have <code>MyClass&lt;T, Alloc&gt;</code>, similar to how, say, STL vector provides. </p> <p>I need this functionality to support an allocator for shared memory. Specifically, I am trying to implement a ring buffer in shared memory. Currently it has the following ctor:</p> <pre><code>template&lt;typename ItemType&gt; SharedMemoryBuffer&lt;ItemType&gt;::SharedMemoryBuffer( unsigned long capacity, std::string name ) </code></pre> <p>where <code>ItemType</code> is the type of the data to be placed in each slot of the buffer.</p> <p>Now, this works splendid when I create the buffer from the main program thus</p> <pre><code>SharedMemoryBuffer&lt;int&gt;* sb; sb = new SharedMemoryBuffer&lt;int&gt;(BUFFER_CAPACITY + 1, sharedMemoryName); </code></pre> <p>However, in this case the buffer itself is not created in shared memory and so is not accessible to other processes. What I want to do is to be able to do something like</p> <pre><code>typedef allocator&lt;int, managed_shared_memory::segment_manager&gt; ShmemAllocator; typedef SharedMemoryBuffer&lt;int, ShmemAllocator&gt; MyBuffer; managed_shared_memory segment(create_only, "MySharedMemory", 65536); const ShmemAllocator alloc_inst (segment.get_segment_manager()); MyBuffer *mybuf = segment.construct&lt;MyBuffer&gt;("MyBuffer")(alloc_inst); </code></pre> <p>However, I don't know how to go about adding an explicit allocator to the class template. </p>
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    1. COI don't quite understand what you are asking. Do you need to know how to add a template parameter to your class template (in which case you probably need to show the current definition of the class), or do you just need to know how to construct an object at a given location in memory (in which case you should search for _placement new_). Either way, I'd be very cautious about putting non-POD objects in shared memory. Is it possible to construct from one process and destroy from another? Are you completely sure that all the class members can handle this?
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    2. COI guess my question can simply be paraphrased as: how can you create your object in shared memory (as opposed to creating it on current process' heap and have it *use* shared memory.)
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    3. COIn this case you need placement new. e.g. if `shm_addr` is a `void*` pointer to shared memory you can do `MyBuffer *pBuf = new (shm_Addr) MyBuffer;` and the new `MyBuffer` will be constructed at the given location.
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