std 将向量 float[3] 移动到向量 class x,y,z

Question

I have a vector of 3 values in two different forms, and I would like to pass from one to the other:

• a std::vector< float[3] > , std::vector< std::vector<float> > or even float[][3] if needed (that's the part I can change)
• a std::vector of the color class

Let's work with these classes:

class color{
    float x,y,z;
    //some methods but no other member
}

What I would like to do:

std::vector<float[3]> myVector; // already allocated and filled
std::vector<color> cv = std::move( myVector ); // of course does not compile

My issue is that I didn't figure out how to convert one to the other without having to reallocate memory. I can convert a float[3] into color using static_cast but it seems it does not work for a vector.

I would like not to iterate over the whole array and simply move the data from myVector inside the cv vector. I thought converting the pointer of myVector to void* and then to color3* could work, but I didn't succeed.

Ideas?

Answer 1

You cannot have a std::vector<float[3]> at all prior to C++20 because float[3] didn't meet the requirements of the value type that std::vector had. Since C++20 it technically is allowed, but is still not useful in practice since any member function that adds elements does still have requirements not satisfied by arrays.

Let's assume that you were asking a similar question where you have two different classes, where one is color and the other contains an array as a member. Such as std::array<float, 3> .

I would like not to iterate over the whole array and simply move the data from myVector inside the cv vector.

There is simply no way to avoid that. The only way to "move" data into a vector without iteration is to move from another vector of the same type. That singular case doesn't apply to your case of different types, so you must have iteration. You don't necessarily need to write the loop yourself, as you may use a standard algorithm instead. Here is an example using standard ranges:

std::vector<std::array<float,3>> myVector; // already allocated and filled
std::vector<color> cv;
auto arr_to_color = [](const std::array<float,3>& arr){
    // assuming such constructor exists
    return color(
        arr[0],
        arr[1],
        arr[2]);
};
std::ranges::transform(
    myVector,
    std::back_inserter(cv),
    arr_to_color);