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POWhy is there no reallocation functionality in C++ allocators?
 

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  1. POWhy is there no reallocation functionality in C++ allocators?
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    <p>In C the standard memory handling functions are <code>malloc()</code>, <code>realloc()</code> and <code>free()</code>. However, C++ stdlib allocators only parallel two of them: there is no reallocation function. Of course, it would not be possible to do exactly the same as <code>realloc()</code>, because simply copying memory is not appropriate for non-aggregate types. But would there be a problem with, say, this function:</p> <pre><code>bool reallocate (pointer ptr, size_type num_now, size_type num_requested); </code></pre> <p>where</p> <ul> <li><code>ptr</code> is previously allocated with the same allocator for <code>num_now</code> objects;</li> <li><code>num_requested</code> >= <code>num_now</code>;</li> </ul> <p>and semantics as follows:</p> <ul> <li>if allocator can expand given memory block at <code>ptr</code> from size for <code>num_now</code> objects to <code>num_requested</code> objects, it does so (leaving additional memory uninitialized) and returns <code>true</code>;</li> <li>else it does nothing and returns <code>false</code>.</li> </ul> <p>Granted, this is not very simple, but allocators, as I understand, are mostly meant for containers and containers' code is usually complicated already.</p> <p>Given such a function, <code>std::vector</code>, say, could grow as follows (pseudocode):</p> <pre><code>if (allocator.reallocate (buffer, capacity, new_capacity)) capacity = new_capacity; // That's all we need to do else ... // Do the standard reallocation by using a different buffer, // copying data and freeing the current one </code></pre> <p>Allocators that are incapable of changing memory size altogether could just implement such a function by unconditional <code>return false;</code>.</p> <p>Are there so few reallocation-capable allocator implementation that it wouldn't worth it to bother? Or are there some problems I overlooked?</p>
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    1. CO+1, this is a question that always bugged me.
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    2. COStroustrup's take on this thing: http://www2.research.att.com/~bs/bs_faq2.html#renew; it delegates the problem to the inner workings of vector, but doesn't say why isn't there a mechanism like "renew" to make the growing of the array simpler.
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    3. COThere is nothing stopping `std::vector` from doing that in some cases (e.g., it knows its using the standard allocator). The standard library is allowed to use knowledge of the underlying system.
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