How do I insert insert into a map of <string, vector<pair:: <string, string>> > in C++?

Question

I am trying to create a symbol table where the key to the map is an identifier and the vector it returns contains pairs of strings representing type and scope, respectively. I am able to get a map with a string key return a vector of single strings to work, but when I try pairs, I get an error.

#include <iostream>
#include <utility>
#include <vector>
#include <map>
#include <string>
using namespace std; //using std namespace for readibility for this question

int main() {
    string key = "myKey";
    string string1 = "string1";
    string string2 = "string2";
    pair <string, string> stringPair = make_pair (string1, string2);

    map<string, vector<std::pair <string, string>>> myMap;
    myMap.insert( make_pair (key,  make_pair (string1, string2) )); //insert key into map, create empty vector
                                    //this is where I get th error

    myMap[key].push_back(std::make_pair (string1, string2) ); //use key, push strings into vector<string, string>

    return 0;
}

error C2664: 'std::pair<_Ty1,_Ty2> std::_Tree<_Traits>::insert(std::pair<const _Kty,_Ty> &&)' : cannot convert parameter 1 from 'std::pair<_Ty1,_Ty2>' to 'std::pair<_Ty1,_Ty2> &&'

I could get a vector of single strings to work, but that seems like more of a workaround than having true pairs for each instance of an identifier. Any help is appreciated!

Klaim · Accepted Answer · 2014-04-07 11:41:27Z

5

Here is a very simple C++11 version that work but might be hard to read later, depends on the people:

#include <iostream>
#include <utility>
#include <vector>
#include <map>
#include <string>
using namespace std; //using std namespace for readibility for this question

int main() {
    string key = "myKey";
    string string1 = "string1";
    string string2 = "string2";
    map<string, vector<std::pair <string, string>>> myMap;

    myMap.insert( { key, {{ string1, string2 }} } ); 

    return 0;
}

Tested there: http://coliru.stacked-crooked.com/ (GCC4.8)

Of course if you want to add to the vector that contain the strings, you need to first check if the vector already exists. Here I am not checking if the vector already exists so if it does, there will be no insersion.

answered Apr 7, 2014 at 11:41

Klaim

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Comments

Chris Dodd · Accepted Answer · 2014-04-07 00:55:48Z

4

The problem is that you're trying to insert a pair into the map, instead of a vector of pairs. If you want the vectors to start empty, the easiest way is to let it use the default constructor (which for a vector, makes an empty vector), and not explicitly insert into the map at all:

map<string, vector<std::pair <string, string>>> myMap;
myMap[key].push_back(std::make_pair (string1, string2) ); //use key, push strings into vector<string, string>

answered Apr 7, 2014 at 0:55

Chris Dodd

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5 Comments

Johnny Over a year ago

Oh, I see my error now. But I still need to insert explicitly to put the key in the map, right?

Chris Dodd Over a year ago

No, map::operator[] constructs a default value and inserts it if the key was not in the map previously. Since vector::vector() (the default ctor) gives an empty vector, that's exactly what you want.

Johnny Over a year ago

Ok, I've got that working. On final question, if you would. How do I access the members in the vector pair? it->first refers to the key, but it->second.first doesn't work.

Chris Dodd Over a year ago

it->second is the vector. So it->second[0].first would be the first string in the first element of the vector

Arne Mertz Over a year ago

@Johnny that's why you should use helper variables with meaningful names: auto const& thevector = it->second; auto const& thepair = thevector[0]; auto const& thestring = thepair.first; (Of course there are better, semantic names). You don't have to break it up that far, but it->second[0].first is (obviously from your question) very obfuscating and hard to get your head around. iow: if it's hard to write, it's harder to read and maintain, so make it easier.

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