CODEWITHSUNDEEP
c++pointersdictionaryreferenceunordered-map
user997112
user997112

Reputation: 30655

map and unordered_map containing pointers to double?

If I have a std::map and std::unordered_map how could I use pointers on the double so that when the unordered_map updates the double value for a particular key, this is already reflected in the std::map for the same "key"?

So:

unordered_map["1"] = 6 causes map["1"] to be 6 also....

Upvotes: 1

Views: 987

Answers (2)

goji
goji

Reputation: 7132

There's no reason why you can't use pointers.

Example:

#include <iostream>
#include <map>
#include <unordered_map>
#include <memory>

int main()
{
    std::unordered_map<std::string, std::shared_ptr<double>> umap;
    std::map<std::string, std::shared_ptr<double>> omap;

    std::shared_ptr<double> value(new double(1234.5));

    umap.emplace("key", value);
    omap.emplace("key", value);

    std::cout << "umap " << *umap["key"] << "\n";
    std::cout << "omap " << *omap["key"] << "\n";

    *umap["key"] = 9999.1;
    std::cout << "omap " << *omap["key"] << "\n";
}

Output:

umap 1234.5
omap 1234.5
omap 9999.1

Upvotes: 4

Kerrek SB
Kerrek SB

Reputation: 477580

Maybe like this:

std::unordered_map<std::string, double> um;
std::unordered_map<std::string, double*> om;

om["1"] = &um["1"];

From now on, *om["1"] is always the value of the corresponding element in um. Just make sure you never delete elements from the unordered map.

(Source: iterator and reference invalidation rules)

Upvotes: 1

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