They way I understand it, references are implemented as pointers under the hood. The reason why you can’t store them in a map is purely semantic; you have to initialize a reference when it’s created and you can’t change it afterward anymore. This doesn’t mesh with the way a map works.
Because of how a map is designed, you’ll need to do the equivalent of a search on unordered data. for (auto it = someMap.begin(); it != someMap.end(); ++it) if (it->second == someValue) return it->first;
int and const int are two distinct types. std::map<int, float> and std::map<const int, float> are, similarly, different types. The difference between std::map<const int, float> and std::map<int, float> is, to a degree, analogous to the difference between, say, std::map<int, float> and std::map<std::string, float>; you get a fresh map type for each. In the non-const case, the … Read more
I suspect you need bool operator<(const Foo& foo1) const; Note the const after the arguments, this is to make “your” (the left-hand side in the comparison) object constant. The reason only a single operator is needed is that it is enough to implement the required ordering. To answer the abstract question “does a have to … Read more
The C++11 standard guarantees that const method access to containers is safe from different threads (ie, both use const methods). In addition, [container.requirements.dataraces] states implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting vector<bool> In other words, except for vector<bool> modifying distinct … Read more
The estimate would be closer to (sizeof(A) + sizeof(B) + ELEMENT_OVERHEAD) * N + CONTAINER_OVERHEAD There is an overhead for each element you add, and there is also a fixed overhead for maintaining the data structure used for the data structure storing the map. This is typically a binary tree, such as a Red-Black Tree. … Read more
You can use boost::multi_index to create a bidirectional map – you can use either value of the pair as a key to do a quick lookup.
Because operator[] returns a reference to the value itself and so the only way to indicate a problem would be to throw an exception (and in general, the STL rarely throws exceptions). If you don’t like this behavior, you can use map::find instead. It returns an iterator instead of the value. This allows it to … Read more
This is plain in C++ 17. Use std::map + std::any + std::any_cast: #include <map> #include <string> #include <any> int main() { std::map<std::string, std::any> notebook; std::string name{ “Pluto” }; int year = 2015; notebook[“PetName”] = name; notebook[“Born”] = year; std::string name2 = std::any_cast<std::string>(notebook[“PetName”]); // = “Pluto” int year2 = std::any_cast<int>(notebook[“Born”]); // = 2015 }
Does not meet the criteria that you have specified, but you could have a look at the TBB containers. There is so called concurrent_hash_map which allows multiple threads to access concurrently the data in the map. There are some details, but everything is nicely documented and can give you an idea of the “concurrent container”. … Read more