Is it possible without copy to pass std::vector<int> as param to fun that get std::vector<std::array<int, 3>>?

Question

I have such a method

void foo(std::vector<std::array<int, 3>> iVec){
....
}

I don't want to change this method, but I need to pass as param this vector std::vector<int>

As far as I understand this two vectors are taking the same memory... So, theoretical I can use method std::move to move this vector as is. But this vectors has a different size.. so... I don't know...

One guy told me that it could be possible if change the function that it will get two int pointers, and so it could be possible.

But I don't understand what does it mean without a example... If someone could provide an example I really appreciate.

But anyway question still is: if is it possible to pass my vector as a function param that has another type. If yes, so how?

Fell free to ask

Answer 1

The C++ standard does not guarantee that sizeof(std::array<MyType, N>)==sizeof(MyType)*N . That is, even though the elements of an individual std::array are contiguous, a vector of array s may not have the elements of the arrays evenly spaced. So there is no standard-compliant way of doing this, because the two vectors would not necessarily be "taking the same memory".

You could maybe get things to work with reinterpret_cast and such on your compiler. ( std::move from a reinterpreted vector type will definitely not work, not in theory and not in practice.) This will not be portable or reliable or a good idea. You have three options: copy the data, change the source type, or change the destination type.

Answer 2

I don't want to change this method ...

But you should. If you make it work with iterators rather than the container, you will get more flexibility:

template <typename IT>
void foo(IT begin, IT end){
    ....
}

Now passing begin and end of whatever container is possible. However, if you want to go back to passing iterators to a std::vector<std::array<int, 3>> you would have to invest some extra effort to get an iterator type that lets you iterate from [0][0] till [size_x][size_y] , but as the arrays all have size 3, this shouldnt be too complicated. And anyhow you should consider to use flat vectors instead of a 2D structure.