C ++转换向量 <pair<double,double> &gt;翻倍*，翻倍*？

Question

Is there a way to get the contiguous memory of the ".first" and ".second" of vector<pair<double,double>> ? What I mean is:

void func(int N, double* x, double* y)
{
    for (int i = 0; i < N; ++i)
        //do something to x[i] and y[i]
}

For the above function I have a vector<pair<double,double>> point rather than vector<double> x, y . I'm guessing this is not possible. If I had a vector x,y then I could do x.data() and y.data() of course.

Answer 1

The memory layouts of std::vector<std::pair<double, double>> xy and std::vector<double> x,y are different. If func is part of a third-party library you can not change, you are bound to

a) call func a couple of times with N=1 or (quick an dirty)

auto xy = std::vector<std::pair<double, double>> {
  {0,0}, {42,0}, {0, 42}, {42, 42}
};
for (auto& [x,y] : xy) { // or for (auto& p : xy) func(1, p.first, p.second)
  func(1, &x, &y);
}

b) convert xy to x and y

template <typename T, typename S>
auto convert(const std::vector<std::pair<T,S>>& xy)
{
  auto xs = std::vector<T>{};
  auto ys = std::vector<S>{};
  xs.reserve(xy.size());
  ys.reserve(xy.size());
  for (auto& [x,y] : xy) {
    xs.push_back(x);
    ys.push_back(y);
  }
  return std::make_pair(xs, ys);
}

int main()
{
  auto xy = std::vector<std::pair<double, double>> {
    {0,0}, {42,0}, {0, 42}, {42, 42}
  };
  auto [x, y] = convert(xy);
  func(xy.size(), x.data(), y.data());
}

c) simply change the defintions of xy to x and y .

If you can change func , I propose to refactor such that you can call the inner loop and rewrite it for iterators (or ranges). That way you could use it with projections on std::pair s.

Here is the full source code.