Question

To get the pointer to the data in a vector we can use

vector<double> Vec;    
double* Array_Pointer = &(Vec[0]);
Function(Array_Pointer);

Is that possible to get the pointer to the data in a set? Can I use that as array pointer like above?

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If not possible, what is the best way to make a vector out of set? I mean without loop over all elements.

Answer 1

No, this is not necessarily possible. The C++ ISO standard explicitly guarantees contiguous storage of elements in a std::vector, so you can safely take the address of the first element and then use that pointer as if you were pointing at a raw array. Other containers in the standard library do not have this guarantee.

The reason for this is to efficiently support most operations on a std::set, the implementation needs to use complex data structures like balanced binary search trees to store and organize the data. These structures are inherently nonlinear and require nodes to be allocated and linked together. Efficiently getting this to work with the elements in a flat array would be difficult, if not impossible, in the time constraints laid out by the standard (amortized O(log n) for most operations.)

EDIT: In response to your question – there is no way to build a std::vector from a std::set without some code somewhere iterating over the set and copying the elements over. You can do this without explicitly using any loops yourself by using the std::vector range constructor:

std::vector<T> vec(mySet.begin(), mySet.end());

Hope this helps!