如何循环通过地图的C ++映射？ [英] How to loop through a C++ map of maps?

问题描述

如何循环使用C ++中的 std :: map ？
我的地图定义为：

  std :: map< std :: string，std :: map< std :: string，std :: string> > 
  

例如，这样保存这样的数据：

  m [name1] [value1] =data1; 
 m [name1] [value2] =data2; 
 m [name2] [value1] =data1; 
 m [name2] [value2] =data2; 
 m [name3] [value1] =data1; 
 m [name3] [value2] =data2; 
  

如何循环遍历此地图并访问各种值？

解决方案

旧问题，但剩下的答案是过时的C ++ 11 - 你可以使用基于循环的范围，只需执行以下操作：

  std :: map< std :: string，std :: map< std :: string，std :: string>> MyMap中; （auto const& ent1：mymap）的
 
 {
 // ent1.first是（auto const& ent2：ent1.second）的
 b $ b // ent2.first是第二个键
 // ent2.second是数据
} 
} 
  

这应该比早期版本更干净，并避免不必要的副本。有些人赞成用引用变量的明确定义来替换注释（如果未使用，则会被优化）：

<$ p （auto const& ent1：mymap）的$ p> {
auto const& outer_key = ent1.first;
auto const& inner_map = ent1.second; （auto const& ent2：inner_map）
{
auto const& inner_key = ent2.first;
auto const& inner_value = ent2.second;
}
}


How would I loop through a std::map in C++? My map is defined as:

std::map< std::string, std::map<std::string, std::string> >

For example, this holds data like this:

m["name1"]["value1"] = "data1";
m["name1"]["value2"] = "data2";
m["name2"]["value1"] = "data1";
m["name2"]["value2"] = "data2";
m["name3"]["value1"] = "data1";
m["name3"]["value2"] = "data2";

How can I loop through this map and access the various values?

解决方案

Old question but the remaining answers are outdated as of C++11 - you can use a ranged based for loop and simply do:

std::map<std::string, std::map<std::string, std::string>> mymap;

for(auto const &ent1 : mymap) {
  // ent1.first is the first key
  for(auto const &ent2 : ent1.second) {
    // ent2.first is the second key
    // ent2.second is the data
  }
}

this should be much cleaner than the earlier versions, and avoids unnecessary copies.

Some favour replacing the comments with explicit definitions of reference variables (which get optimised away if unused):

for(auto const &ent1 : mymap) {
  auto const &outer_key = ent1.first;
  auto const &inner_map = ent1.second;
  for(auto const &ent2 : inner_map) {
    auto const &inner_key   = ent2.first;
    auto const &inner_value = ent2.second;
  }
}

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