How can I loop through a C++ map of maps?

Question

How can 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, the above container 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?

Answer 1

Old question but the remaining answers are outdated as of C++11 - you can use a ranged based for loop and simply do: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