是否有任何标准功能可以创建所有对的范围/视图?以下代码说明了我要创建的视图:
std::unordered_map<std::string, std::vector<int>> m{{"Foo", {1,2}}, {"Hello", {4,5}}};
auto view = ???;
std::vector<std::pair<std::string, int>> v{view.begin(), view.end()};
std::vector<std::pair<std::string, int>> out1{{"Foo", 1}, {"Foo", 2}, {"Hello", 4}, {"Hello", 5}};
std::vector<std::pair<std::string, int>> out2{{"Hello", 4}, {"Hello", 5}, {"Foo", 1}, {"Foo", 2}};
assert(v == out1 || v == out2);
注意:编写一个嵌套的 for 循环来迭代此结构很简单。
在C++23中你可以使用
views::cartesian_product
:
std::unordered_map<std::string, std::vector<int>> m{
{"Foo", {1,2}}, {"Hello", {4,5}}};
auto view = m | views::transform([](auto& p) {
return views::cartesian_product(views::single(p.first), p.second); })
| views::join;
std::ranges::views::join
与 std::ranges::views::transform
和一些 lambda 结合起来。
using namespace std::ranges::views;
auto to_pairs = [](auto & pair){
auto to_pair = [&key=pair.first](auto & value){ return std::pair{ key, value }; };
return pair.second | transform(to_pair);
};
auto view = m | transform(to_pairs) | join | common;