How do I pass a map as a parameter and add to it in this method?

Question

So I have a map

map<string, string> myMap;

SetMapPairs(map);

void SetMapPairs(map<string, string> mapPairs)
{  
    map<string, string> myMap = mapPairs;
    myMap["one"] = "two";
}

I know that I'm doing it wrong but I'm not sure how to do it.
How can I pass it by reference so that I can add to the map in this method?
Also I need to first set myMap = mapPairs otherwise I know it's easy to do
void SetMapPairs(map<string, string> &mapPairs)

Solution 1

Use & to pass by reference:

void SetMapPairs(std::map<std::string, std::string>& mapPairs)
{
    // ...
}

Solution 2

typedef std::map<std::string, std::string> MyMap;


void myMethod(MyMap &map)
{
    map["fruit"] = "apple";
}

or

void myMethod(const MyMap &map)
{
    //can't edit map here
}

Solution 3

You use & to pass by reference:

void SetMapPairs(map<string, string> & mapPairs)
{                                 // ^ that means it's a reference
    mapPairs["one"] = "two";
}

Solution 4

At least for this particular case, I think I'd probably return a map instead of passing one in by reference:

map<string, string> SetMapPairs() {
    std::map<string, string> temp;

    temp["one"] = "two";
    return temp;
}

Then in your calling code, you can use something like:

map<string, string> MyMap = SetMapPairs();

With most decent/modern compilers the generated code will end up about the same either way, but I think under the circumstances, this is a better fit for what you're really doing.

View more solutions

Comments

[juanchopanza]

Are you really calling SetMapPairs(map);? That shouldn't even compile.

[Pittfall]

I tried that first and my map ends up empty after I finish with the method call

[Pittfall]

I need to first set myMap to the mapPairs parameter because this will eventually be a parameter in a constructor

[Pittfall]

sorry, I thought the detail I provided was enough, I should've just said the whole thing. The reason I need to set it is because that SetMapPairs() method will actually be a constructor that takes a map and that's why I can't have it as a return type method

[Jerry Coffin]

@Pittfall: That doesn't make much sense. A ctor should construct the object to which it belongs, not something you send to it as a parameter.

[Pittfall]

yeah, maybe I need to re-think my design