for_each

C++11引入了range for loop,可是range for loop只能拿到向前range的元素，而没法取得循环的当前下标。

folly提供了for_each算法，相比标准库的for_each，folly的for_each回调的函数对象或函数可接受两个参数，这时第一个指当前元素，第二个值循环的下标。同时folly的for_each还可对异构的tuple操作。此外folly for_each还可支持提前结束操作，类似for循环的break。

例子：

#include <gtest/gtest.h>
#include <folly/container/Foreach.h>
#include <type_traits>
#include <tuple>
#include <vector>
#include <fmt/format.h>
TEST(for_each, foreach)
{
  auto v = std::vector {1,2,3};
  auto t = std::make_tuple (1,2,3);
  auto tb = std::make_tuple (1,2,3.2);
  auto func = [](auto elem,auto index){
    fmt::print("elem:{}, index: {}\n", elem,index);
    if (elem == 2)
    {
      return folly::loop_break;
    }
    return folly::loop_continue;
  };
  auto funcb = [](auto elem){
    fmt::print("elem:{}\n", elem);
    if (elem == 2)
    {
      return folly::loop_break;
    }
    return folly::loop_continue;
  };
  folly::for_each (v,func);
    fmt::print("++++++\n");
  folly::for_each (t,func);
    fmt::print("++++++\n");
  folly::for_each (tb,func);
    fmt::print("++++++\n");
  folly::for_each (tb,funcb);
    fmt::print("++++++\n");

}

Posted 2023-05-20

algorithm