Question

I am trying to have a private instance variable and use a getter method that returns a reference to that private ivar (I know I can just make the ivar public).

When I modify the variable after using the getter methods, it seems to be modifying a copy and of the ivar not the original. Any ideas why?

#include <iostream>
#include <tr1/unordered_map>
#include <tr1/functional>
#include <tr1/utility>

typedef std::tr1::unordered_map<std::string, std::string> umap_str_str;

class Parent {
public:

    //add an item to the private ivar
    void prepareIvar(bool useGetter)
    {
        std::pair<std::string, std::string> item("myKey" , "myValue");

        if(useGetter){
            //getting the reference and updating it doesn't work
            umap_str_str umap = getPrivateIvar();
            umap.insert( item );
        }else {
            //accessing the private ivar directly does work
            _UMap.insert( item );
        }

    }
    void printIvar()
    {
        std::cout << "printIvar\n";
        for( auto it : _UMap){
            std::cout << "\tKEY: " << it.first << "VALUE: " << it.second << std::endl;
        }
    }

    //get a reference to the private ivar
    umap_str_str& getPrivateIvar()
    {
        return _UMap;
    }
private:
    umap_str_str _UMap;
};



int main(int argc, const char * argv[])
{
    Parent *p = new Parent();

    p->prepareIvar(true);//use the getter first
    p->printIvar();//it doesn't print the right info

    p->prepareIvar(false);//access the private ivar directly
    p->printIvar();//prints as expected


    return 0;
}

Answer 1

In this line, you are using the getPrivateIvar() method, which returns a reference. However, you are storing it in a variable of type umap_str_str:

umap_str_str umap = getPrivateIvar();

What is happening is that you are creating a new umap_str_str object, which will be a copy of the _UMap private member. You need to use a reference instead:

umap_str_str &umap(getPrivateIvar());