CODEWITHSUNDEEP
c++standard-librarystd-pair
Fatima
Fatima

Reputation: 1689

Std Pair Initialization

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.

 };

Upvotes: 2

Views: 12273

Answers (3)

Rapptz
Rapptz

Reputation: 21317

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; 
    }

 };

Upvotes: 3

Joseph Mansfield
Joseph Mansfield

Reputation: 110658

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));

Upvotes: 4

Kerrek SB
Kerrek SB

Reputation: 477010

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);

Upvotes: 1

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