如何在 C++ 中迭代谷歌 protobuf 地图？

Question

I have my map in the following form:

const google::protobuf::Map<string, nrtprofile::RedisNewsMetric> map=redisNewsMessage.ig();

How should I iterate through the map to get all the keys and corresponding values?

Answer 1

You iterate through a google::protobuf::Map in exactly the same way as a std::unordered_map .

for (auto & pair : map)
{
    doSomethingWithKey(pair.first);
    doSomethingWithValue(pair.second);
}

If you have a C++17 compiler, you can use a structured binding to split that further

for (auto & [key, value] : map)
{
    doSomethingWithKey(key);
    doSomethingWithValue(value);
}

Answer 2

From the documentation

A google::protobuf::Map is a special container type used in protocol buffers to store map fields. As you can see from its interface below, it uses a commonly-used subset of std::map and std::unordered_map methods

and

google::protobuf::Map supports the same iterator API as std::map and std::unordered_map. If you don't want to use google::protobuf::Map directly, you can convert a google::protobuf::Map to a standard map by doing the following:

So either of the two approaches shown in the example code below should work:

int main ()
{
  std::map<char,int> mymap;

  mymap['b'] = 100;
  mymap['a'] = 200;
  mymap['c'] = 300;

  // show content:
  for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
    std::cout << it->first << " => " << it->second << '\n';

  for (auto& x : mymap)
    std::cout << x.first << " => " << x.second << '\n';

  return 0;
}

Answer 3

to be able to do structured binding for with protobuf Map:

for (auto & [key, value] : map) {
}

you need to include this bit of code that tells the compiler to to do it:

namespace std {
    template<typename TK, typename TV>
    class tuple_size<google::protobuf::MapPair<TK,TV>> : public std::integral_constant<size_t, 2> { };

    template<size_t I, typename TK, typename TV> 
    struct tuple_element< I, google::protobuf::MapPair<TK, TV>> { };
    template<typename TK, typename TV> struct tuple_element<0, google::protobuf::MapPair<TK, TV>> { using type = TK; };
    template<typename TK, typename TV> struct tuple_element<1, google::protobuf::MapPair<TK, TV>> { using type = TV; };

    template<int I, typename TK, typename TV>
    auto get(const google::protobuf::MapPair<TK, TV>& x) {
        if constexpr (I == 0) return x.first;
        if constexpr (I == 1) return x.second;
    }
}