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    <p>In essence, when you use a new expression like: <code>T *t = new T;</code>, it's roughly equivalent to:</p> <pre><code>void *temp = operator new(sizeof(T)); T *t = new(temp) T; </code></pre> <p>So, first it allocates some raw memory using the allocation function, then it constructs an object in that memory. Likewise, when you use a delete expression like: <code>delete t;</code>, it's roughly equivalent to:</p> <pre><code>t-&gt;~T(); operator delete(t); </code></pre> <p>So, if you overload <code>new</code> and <code>delete</code> for a particular class:</p> <pre><code>class T { int data; public: // I've made these static explicitly, but they'll be static even if you don't. static void *operator new(size_t size) { return malloc(size); } static void operator delete(void *block) { free(block); } }; </code></pre> <p>Then when you use a new expression, it'll invoke the class' <code>operator new</code> to allocate the memory, and that will call <code>malloc</code>, so <code>T *t = new T();</code> will end up allocating memory via <code>malloc</code> (and likewise, when you <code>delete</code> it, it'll use <code>operator delete</code>, which will call <code>free</code>).</p> <p>At least as the term is normally used, an Allocator is quite similar, except that it's used by a container instead of other code. It also encapsulates the allocation function and deletion function into a class, so when you pass one to the container, you only have to pass one object, and there's little chance of an allocation and delete function getting mismatched.</p> <p>Ignoring, for the moment, the details about what names are used for things, the Allocator class in the standard library <em>mostly</em> does the same, so with a little renaming of the functions in the <code>T</code> class above, you'd be about half done writing a standard allocator. To go with the allocation and deletion, it has a function to <code>rebind</code> some memory (change a block of memory to another type), create an object in place (basically just a wrapper around a placement new) and destroy an object (again, trivial wrapper around destructor invocation). Of course, it uses <code>operator new</code> and <code>operator delete</code> instead of <code>malloc</code> and <code>free</code> like I've used above.</p>
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    1. COIf placement new is what I need to use, then how come for example MSVC's `new` does not call placement new in its version of `new` (in new.cpp, you can get it by stepping into a new call).
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    2. CO@mmurphy: *Roughly* similar to, definitely not identical to.
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