unordered_map key issue "expression must have integral or unscoped enum type"

Question

I have this:

unordered_map<string,string>* createDict(){

    unordered_map<string,string>* Dict= new unordered_map<string,string>();

    Dict["x"] = "a";
    Dict["y"] = "b";
    Dict["z"] = "c";

    return Dict;
}

and I get the error:

expression must have integral or unscoped enum type

but this works when allocating on the stack. What is the easiest way to solve this? I really dont like having to create Pair objects to insert in to an unordered_map.

Why not have the function return an unordered_map instead of a pointer to one? — us2012
– us2012, Commented Sep 20, 2013 at 22:10
@us2012 I must be honest I was going to do that, but this is going to be a data member of a class. — user997112
– user997112, Commented Sep 20, 2013 at 22:11
@user997112 Why does being a class' data member make it necessary to dynamically allocate the map? — Praetorian
– Praetorian, Commented Sep 20, 2013 at 22:13
@user I'm wondering whether you might have some misconceptions about what it means to have an object member variable vs a pointer member variable in a class A. You do not need a pointer member variable just because you want to dynamically allocate objects of type A! — us2012
– us2012, Commented Sep 20, 2013 at 22:32

Dietmar Kühl · Accepted Answer · 2013-09-20 22:09:58Z

4

Dict is a pointer to an std::unordered_map<std::string, std::string> but you are using it as if it is an object. You need to dereference the object appropriately, e.g., using

(*Dict)["x"] = "a";
Dict->operator[]("y") = "b";
std::unordered_map<std::string, std::string>& ref = *Dict;
ref["z"] = "c";

I'd prefer the first option or use

Dict->insert(std::make_pair("x", "a"));

Which has, however, slightly different semantics: if the key exists, it doesn't do anything while using the subscript operator and assignment would update the value.

answered Sep 20, 2013 at 22:09

Dietmar Kühl

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score 1 · Accepted Answer · 2013-09-20 22:23:45Z

1

What is the easiest way to solve this?

Use universal initialization and just return the unordered_map rather than a pointer:

#include <unordered_map>
#include <string>

std::unordered_map<std::string, std::string> createDict(){
    return {{"x", "a"}, {"y", "b"}, {"z", "c"}};
}

int main() {
  auto xyz_dict = createDict();
}

Edit: Based on your comment that this data structure will be a member variable of some class, you could just initialize it in class:

struct A {
  using xyzDict = std::unordered_map<std::string, std::string>;
  xyzDict xyz_dict{{"x", "a"}, {"y", "b"}, {"z", "c"}};   
};

If your compiler doesn't support this, then just initialize xyz_dict with createDict() in the constructor's member initialization list.

edited Sep 20, 2013 at 22:23

answered Sep 20, 2013 at 22:16

user425495

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