Passing c map by reference and see changes after insert-CodePudding

Hey guys!

I'm just getting into c and after learning stuff about reference and value pass I've come across a problem.

So basically I'm trying to copy a map and I do so in my method. So far the size of the 2 maps are the same.

The problem comes when I insert some new values into the original map because it doesn't change the copied map.

So my question is how do I copy/pass a map, with the new map being a real copy and when the original changes, the copied version does so.

I'll attach the code that I was working on.

The code:

#include <iostream>
#include <map>

using namespace std;

map<string, int> passMapByReference(map<string, int>& temp_map){
 return temp_map;   
}

void printMap(map<string, int>& temp_map ){
    cout << temp_map.size() << endl;
}

int main()
{
    map<string, int> copyMap;
    
    map<string, int> map;
    map["asd"] = 1;
    map["dsa"] = 2;
    
    printMap(map);

    copyMap = passMapByReference(map);
    
    printMap(copyMap);
    
    map["ksdbj"] = 3;
    map["askdnijabsd"] = 4;
    
    printMap(map);
    
    //this should print 4
    printMap(copyMap);
    

    return 0;
}

The output:

CodePudding user response：

map and copyMap are separate objects and so changing one won't affect other. Moreover, you're returning the map by value from the function passMapByReference. Instead you could return the map by reference from passMapByReference as shown below:

#include <iostream>
#include <map>
#include <string>

//------------------------v------->return by reference
std::map<std::string, int>& passMapByReference(std::map<std::string, int>& temp_map){
 return temp_map;   
}

void printMap(std::map<std::string, int>& temp_map ){
    std::cout << temp_map.size() << std::endl;
}

int main()
{  
    std::map<std::string, int> map;
    map["asd"] = 1;
    map["dsa"] = 2;
    
    printMap(map);

    //copyMap is an lvalue reference to passMapByReference
    std::map<std::string, int>& copyMap = passMapByReference(map);
    
    printMap(copyMap);
    
    map["ksdbj"] = 3;
    map["askdnijabsd"] = 4;
    
    printMap(map);
    
    printMap(copyMap);//prints 4
    

}

Working demo

The output of the above program is:

Note

I noticed that you have used using namespace std; and then created a map with the name map. This should be avoided as it creates confusion. For example, it becomes hard to see whether the map you're referring to is a std::map or the variable named map. I would recommend using std:: to qualify the standard contianers instead of using using namespace std;.

Refer to Why is "using namespace std;" considered bad practice?