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    <p><code>delete []</code> does know the size that was allocated. However, that knowledge resides in the runtime or in the operating system's memory manager, meaning that it is not available to the compiler during compilation. And <code>sizeof()</code> is not a real function, it is actually evaluated to a constant by the compiler, which is something it cannot do for dynamically allocated arrays, whose size is not known during compilation.</p> <p>Also, consider this example:</p> <pre><code> int *arr = new int[256]; int *p = &arr[100]; printf("Size: %d\n", sizeof(p)); </code></pre> <p>How would the compiler know what the size of <code>p</code> is? The root of the problem is that arrays in C and C++ are not first-class objects. They decay to pointers, and there is no way for the compiler or the program itself to know whether a pointer points to the beginning of a chunk of memory allocated by <code>new</code>, or to a single object, or to some place in the middle of a chunk of memory allocated by <code>new</code>.</p> <p>One reason for this is that C and C++ leave memory management to the programmer and to the operating system, which is also why they do not have garbage collection. Implementation of <code>new</code> and <code>delete</code> is not part of the C++ standard, because C++ is meant to be used on a variety of platforms, which may manage their memory in very different ways. It may be possible to let C++ keep track of all the allocated arrays and their sizes if you are writing a word processor for a windows box running on the latest Intel CPU, but it may be completely infeasible when you are writing an embedded system running on a DSP.</p>
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    1. POIs there any way to determine the size of a C++ array programmatically? And if not, why?
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    1. POIs there any way to determine the size of a C++ array programmatically? And if not, why?
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    1. COGood explanation Dima.
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      1. USJorge Ferreira
    2. COThere are absolutely arrays in C++. Howe else would you explain why with this "char x[4]; size_t sz = sizeof(x);" that 'sz' will be assigned 4?
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      1. USKevin
    3. COKevin, x is not really an array. It is a pointer to a chunk of memory, which happens to be on the stack. And because it was allocated statically on the stack, the compiler knows its size. If you pass x to a function, it would not know its size. Arrays in C or C++ aren't first-class objects.
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