Emplace to std::map of std::map

Question

How can I emplace to std::map<std::string, std::map<std::string,std::string>>?

tried with myMap.emplace(std::make_pair("STRING", std::make_pair("STR","STR"))) but got error message cannot convert std::pair<_Ty1,_Ty2> to const std::pair<_Ty1,_Ty2>

`myMap["STRING] = {{"STR", "STR"}};` :-) – Kerrek SB Feb 24 '17 at 10:50 — Kerrek SB, Feb 24 '17 at 10:50

Nikita Smirnov · Answer 1 · 2017-02-24T10:13:55.003

1

myMap.emplace(
      std::make_pair(
            "STR1"
         ,   std::map< std::string, std::string>(
                {
                   std::make_pair("STR2", "STR3")
                }
             )
      )
);

Should work.

edited Feb 24 '17 at 10:13

answered Feb 24 '17 at 10:09

Nikita Smirnov

813
8
17

Did you included ? – Nikita Smirnov Feb 24 '17 at 10:24
A little more weird code then: `(std::initializer_list>({ std::make_pair("STR2", "STR3") }))` in nested map's constructor, if first variant doesn't really work. – Nikita Smirnov Feb 24 '17 at 10:27
1

Initializer lists are not supported by VS2012 :-( – leon22 Feb 24 '17 at 10:27
Ah, yeap, I didn't noticed tag( Then I have another bad idea, but it's not a real emplacing - create nested map outside, and just pass as parameter to first std::make_pair. And for only interest - here is C++'17 coming, and VS2012 is still actual?) – Nikita Smirnov Feb 24 '17 at 10:31
In that environment I have to use the old compiler, but nevertheless I will use insert instead of emplace! Thx – leon22 Feb 24 '17 at 11:41

score 0 · Answer 2 · edited Jan 28 '19 at 10:02

Using std::piecewise_construct you can emplace() as follows

   myMap.emplace(
      std::piecewise_construct,
      std::forward_as_tuple("STRING"),
      std::forward_as_tuple(
         std::initializer_list<std::pair<std::string const, std::string>>{
            {"STR", "STR"} }));

but I think it's a little clearer emplace an empty map and immediatly after emplace the "STR"s in it, as follows

   myMap.emplace(std::piecewise_construct,
                 std::forward_as_tuple("STRING"),
                 std::forward_as_tuple()).first->second.emplace("STR", "STR");

Emplace to std::map of std::map

2 Answers2