Std Pair Initialization

Question

This is my first time working with pairs, totally confused. How to initialize a pair as to insert it in the map?
Should I include some standard library for this?

#include <string>
#include <map>
using namespace std;
class Roads
{  
 public:  
  map< pair<string,string>, int > Road_map; 
  void AddRoad( string s, string d )
       { int b = 2 ; Road_map.insert( pair<s,d>, b) ; }  //pair<s,d> is wrong here.

 };  

Solution 1

You can use std::make_pair:

Road_map[make_pair(s, d)] = b;

Alternatively, you can construct an std::pair like so:

Road_map[pair<string,string>(s,d)] = b;

The std::make_pair approach saves you having to name the types of s and d.

Notice that the appropriate function here is operator[] and not insert. std::map::insert takes a single argument which is a std::pair containing the key and value you want to insert. You would have to do that like this:

Road_map.insert(pair<const pair<string,string>, int>(make_pair(s, d), b);

You can make this a bit prettier with typedef:

typedef map<pair<string,string>, int> map_type;
Road_map.insert(map_type::value_type(map_type::key_type(s, d), b));

Solution 2

Use std::make_pair instead. Like so:

#include <string>
using namespace std;
class Roads
{  
 public:  
    map< pair<string,string>, int > Road_map; 
    void AddRoad( string s, string d )
    { 
        int b = 2 ; 
        Road_map[make_pair(s,d)] = b; 
    }

 }; 

Solution 3

For a map<K, T>, the value_type is actually pair<K const, T>. However, the easiest way to access this is by using typedefs:

typedef std::pair<std::string, std::string> string_pair;
typedef std::map<string_pair, int>             map_type;

// ...

Road_map.insert(map_type::value_type(map_type::key_type(s, d), b));

In C++11 you can use the easier emplace interface:

Road_map.emplace(map_type::key_type(s, d), b);

Comments

[Joseph Mansfield]

Am I going crazy or will this not work because insert is supposed to take a single argument of value_type (pair with key and value)? There's no insert that takes key and value arguments.

[Fatima]

This didn't work. It gave the following error: cannot convert parameter 1 from 'std::pair<_Ty1,_Ty2>' to 'std::_Tree<_Traits>::const_iterator' What should I use in this case ?

[Rapptz]

@sftrabbit you are correct. Interestingly enough I didn't look at it enough.

[Joseph Mansfield]

@Ever Yeah, I thought so. It's trying to call the two argument version of insert which takes a const_iterator and some value.

[Fatima]

@sftrabbit Okay, what should I use in this case ? Is there an alternative for insert?

[Rapptz]

@Ever look at my edit. You can use myMap[key] = value; syntax. Just like other answers have provided.