C++ 嵌套映射没有匹配的成员函数 const 成员

Question

I have a nested map, something like

map = {
  key : {
    innerKey: innerVal
  }
}

I'm trying to search for innerVal from member function marked as const . I'm using at() as described here C++ map access discards qualifiers (const) which gets me to map pointed by key . But, when I try to use at() on nested map, I get an error:

error: no matching member function for call to 'at'

Workaround: I can use a iterator and linearly search on the nested map, which works perfectly. How can I use functions such as at() or find() to search in nested map.

TLDR:

private std::map<int, std::map<int, int> > privateStore;

int search(int key1, int key2) const {
  return privateStore.at(key1).at(key2); //works when I remove `const` from function signature

}

Edit: it works for above simplified code, try this , and try removing const keyword from line 20.

#include <iostream>
#include <map>
#include <thread>

template <typename T>
class Foo
{
public:
  Foo()
  {
    std::cout << "init";
  }

  void set(T val)
  {
    privateStore[std::this_thread::get_id()][this] = val;
  }

  T search(std::thread::id key1) const
  {
    std::map<Foo<T>*, T>  retVal = privateStore.at(key1); //works when I remove `const` from function signature
    return retVal.at(this);
  }

private:
  static std::map<std::thread::id, std::map<Foo<T>*, T> > privateStore;
};

template<typename T> std::map<std::thread::id, std::map<Foo<T>*, T> > Foo<T>::privateStore = {};

int main()
{
  Foo<int> a;
  a.set(12);
  std::cout << a.search(std::this_thread::get_id());
}

Answer 1

Declare your inner map's key to be a pointer to const object. Otherwise when you pass this in a const function you pass Foo<T> const* instead of Foo<T>* and you can't convert that implicitly.

So

static std::map<std::thread::id, std::map<Foo<T> *, T> > privateStore;

to

static std::map<std::thread::id, std::map<Foo<T> const*, T> > privateStore;

And the same in the definition.

live example of your example - fixed.