let's say I have:

namespace name{
    template< typename T >
    class Example
    {
    .... 
    };
}

I want to have a single instance of a map that I'll use across all the classes that inherit from Example. The first thing that came into my mind was having a static member of Example but it will force me to have a unique member for each class that will use it - and I don't want that due to memory usage restrictions (Am I wrong here?).

Declaring it in the namespace will also create an instance for each compilation unit.

What can I do to overcome this?

BTW my initial solution was to [use a const extern map][1] but I can't understand the error I'm getting there.

EDIT:

I've followed @SamVarshavchik answer, and now I'm getting the same error as in the previous question.

namespace name{

    struct Example_base {
    protected:
           static std::map<std::string, int> the_same_map;
    };

    template< typename T >
    class Example : public Example_base
    {
    .... 
    };
}

Results

R_X86_64_PC32 relocation at offset 0xd3 against symbol `name::Example_base::map' can not be used; recompile with -fPIC

For each usage in every compilation unit [1]: R_X86_64_PC32 relocation when using a const extern map

Note that I've multiple instances of the classes that implement Example with the same template and that I can't use this flag

CodePudding user response：

This calls for inheritance.

namespace name{

    struct Example_base {
    protected:
           static std::map<std::string, int> the_same_map;
    };

    template< typename T >
    class Example : public Example_base
    {
    .... 
    };
}

Now, all subclasses of any instance of the Example template share the_same_map.