Initialize array from pointer and size of another array

Question

In a function, I have access to the pointer and size of another array. Passing these along with the pointer, I want to add that data to a map. How can I do that without copying the underlying data twice?

void addToMap(double* ptr, size_t size, unsigned key) {

 // Here I want to add the array corresponding to the pointer location and size to the map.
 // Since adding to the map needs to copy the data once anyway, I want to prevent any other copies.
 double array[size] = ?;

 // Can I initialize this array from the data found at (ptr) to (ptr + size) without a copy?
 array& = ptr; 

 _map[key] = array;

}

The data being pointed to is deleted after this function is called. The map is just a map from an integer to an array of doubles. That simply means, I want to somehow store the data associated with this key while it is still available.

Or do you have a better idea of doing this? Would using a std::vector<double> slower here?

double array[size] isn't standard C++ anyways. Size of the array must be compile time constant. — Algirdas Preidžius
– Algirdas Preidžius, Commented Apr 22, 2021 at 13:33
Is using array necessary? Isn't simply using pointer like double* array = ptr; not accepted? — MikeCAT
– MikeCAT, Commented Apr 22, 2021 at 13:33
What is _map that you would try to assign to _map[key] by passing an array? You can't do what you are asking, but there may be other solutions depending on what _map is. — François Andrieux
– François Andrieux, Commented Apr 22, 2021 at 13:33
array in _map[key] = array; will automatically converted to a pointer anyway. — MikeCAT
– MikeCAT, Commented Apr 22, 2021 at 13:34
std::vector<double> array(ptr, ptr + size);. That also means you map needs to have std::vector<double> as the value type — NathanOliver
– NathanOliver, Commented Apr 22, 2021 at 13:34

eerorika · Accepted Answer · 2021-04-22 14:10:06Z

1

You cannot define an array variable like that because the function argument can vary at runtime, and is therefore not compile time constant which is a requirement for the size of an array variable.

You'll need to allocate the array dynamically. std::vector is the simplest way to do that.

An array can also not be directly initialised from a pointer to another array, whether you're initialising an array variable (which isn't an option here) or a dynamic array. Here too, std::vector helps with the constructors that it offers.

How would I use std::vector<double> here then?

Example:

std::unordered_map<unsigned, std::vector<double>> _map;

// addToMap
_map.emplace(key, std::vector<double>{ptr, ptr + size});

I recommend using std::span<double> as the argument instead of the pointer and size.

edited Apr 22, 2021 at 14:10

answered Apr 22, 2021 at 13:41

eerorika

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5 Comments

EigenGrau Over a year ago

How would I use std::vector<double> here then?

EigenGrau Over a year ago

Thanks! Seems to be working. And this only copies the data once?

eerorika Over a year ago

@EigenGrau Yes.

EigenGrau Over a year ago

I recommend using std::span<double> as the argument instead of the pointer and size. Why?

eerorika Over a year ago

@EigenGrau It's simpler to use. That way you could for example call: addToMap(some_array, key) instead of addToMap(some_array, std::size(some_array), key). Another alternative would be to accept a templated range by value which would allow avoiding a copy entirely if the caller passes an rvalue std::vector<double>.

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