c++ why isn't there something like length(array)? [closed]

Question

Well I don't think that it's really important but since the program has to store the length because of delete[] anyway, Why can't we get this "stored information" ?

Answer 1

The implementation only needs to store the length, and typically only does, if the type is not trivially destructible (i.e., it needs to generate calls to a destructor) and the array was created with the new[] operator.

Since that property of the arrayed type bears no relation to the size of the array, it is more elegant simply to call the length "cookie" a private implementation detail.

To get the length of a complete array object (not a mere pointer), you can use std::extent< decltype( arr ) >::value or std::end( arr ) - std::begin( arr ).

Using new[] with a class with a destructor is a code smell. Consider std::vector instead. The overhead vs raw new[] (considering all bytes that need to be allocated, wherever they are) is one pointer's worth of bytes, and the benefits are innumerable.

Answer 2

Consider the case:

char* a = new char[100];

Now a needs to point to a buffer that's at least 100 chars big, but the system might have allocated a bigger buffer to fulfill this.

With this in mind, we can see that the system is free to immediately forget the size the program asked for, as long as it can still deallocate the buffer properly later. (Either by remembering the size of the allocated buffer or doing the memory allocation with some smart data structure where only the pointer to the start is required)

So, in the general case, the information you are looking for is not, in fact, stored anywhere.