Have a map with keys consisting of different types in C++

问题

I currently am using a map in C++ and have a couple of values that are ints and booleans, though the majority of them are strings. I know in Java I could do something like this:

std::map<string, Object*> mapvar;

but is there an Object equivalent in C++?? If not, is there any way I could properly have values being a std::string, int, or bool?

回答1:

I don't know of a way you could have a std::map with keys that can be multiple types using the stdlib out of the box.

Boost has a variant type that functions more or less like what you want.

typedef boost::variant<std::string, int, bool> key_type;
std::map<key_type, Object*> mapvar

This map will only accept keys that are defined in your chosen key_type, so you could not also use, say, a std::wstring as a key without explicitly declaring it. This would be the advantage of using a variant over any. The latter might happily eat whatever type you send its way, while the former will give you a compile time error.

If you are going to use a boost::variant as your key then you need to read the boost documentation and look into what is involved in constructing variant objects and whether or not they have sorting pre-defined. std::map is built on operator< (unless you template it on some other sorting functor) so you'll need to make sure a working definition for your variant type is provided by either the library or yourself.

--

Why do you want to have such a map in the first place?

回答2:

You can have several different maps, and hide the lookup in overloaded functions:

std::map<int, Object>           map_int;
std::map<std::string, Object>   map_string;

Object& lookup(int Key)
{ return map_int[Key]; }

Object& lookup(std::string Key)
{ return map_string[Key]; }

Now all your lookups kan be done with lookup(key).

回答3:

"but is there an Object equivalent in C++??"

No, there isn't such thing in standard C++. c++-cli may provide something similar perhaps.

"If not, is there any way I could properly have keys being a string, int, or boolean?"

There's stuff like boost::variant or boost::any, but I'm not sure, if you could use them as a key in a std::map.

Sounds a bit you're asking for a XY-Problem. Explain your actual use case please.

来源：https://stackoverflow.com/questions/31993839/have-a-map-with-keys-consisting-of-different-types-in-c