C++11 and above OOP Posts

What is a Tuple, a Pairs and a Tie in C++

June 17, 2021
Deepti
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What are Tuples?

In C++ language, the classes work as user defined data types. The class contains both data and function together. Therefore, to make the data more and more organized, C++11 has introduced a new concept of tuple. Tuples give very good flexibility for creating a collection of different or same data types.

In a class, we can access a data member or function using its name. However, in a tuple we use get<index> helper function for the same. Undoubtedly, in Object Oriented programming, the Tuple concept provides much more power for creating and handling multiple user defined data.

Header file required

#include<tuple>

How to define tuple data type

std::tuple<int,int> t1 (5,5); 
            // t1 tuple holding 2 int values

std::tuple<int,char,float>t2(5,'a',34.3); 
           // t2 tuple holding int, char and float values

std::tuple<char,string,float>t3('a',"MainFunda".23.80);  
          // t3 tuple holding char, string and float

Basic Example of tuple in C++

Lets us create a Sample application for seeing how to set and get data from tuples

#include <iostream>  //Main header file
#include <string>   // for string
#include <tuple> //for tuple

using namespace std;

int main()
{    
  std::tuple<int, float, char,std::string> t1 
                      (5,23.8,'a',"MainFunda");   
             //created tuple holding 4 data values	
    
    int iData = std::get < 0 > (t1);  
            // get 1st int value
     
    float fData= std::get < 1 > (t1);  
            // get 2nd float value
   
    char cData = std::get<2>(t1);  
            // get 3rd char vaue
	
    std::string strData = std::get < 3 > (t1);   
           // get 3rd char value 	
	
    cout <<"Int is "<< iData
         <<"\nFloat is  "<<fData
         <<"\nChar is  "<<cData
         <<"\nString is  "<<strData
         <<endl;

    return 0;
}

As seen above getting data from a tuple object is very easy . We just need to call get<index-1> for knowing the content of data from tuple.

output of code using a tuple in c++

Error scenario with invalid get<index>

Lets see one more application where we are calling get<index-1> with invalid index 

#include <iostream>  //Main header file
#include <string>   // for string
#include <tuple> //for tuple

using namespace std;

int main()
{    
   std::tuple<int, float, char,std::string> t1 
               (5,23.8,'a',"MainFunda");   
              //created tuple holding 4 data values
	
    
   int iData = std::get < 5 > (t1);  
              // get 5th (invalid) value
     
	
	
   cout <<"Int is "<< iData
        <<endl;

    return 0;
}

We are getting compile time error for accessing invalid index for tuple. This is the advantage of tuple .

compiler error on using wrong index in std:;get(index) with tuples

Tuple – Helper Function

get( )

The function get( ) takes an integer index as input and returns the reference to tuple data member at that index . In turn, we can use such reference for setting a tuple data member also. Below application demonstrates this

#include <iostream>  //Main header file
#include <string>   // for string
#include <tuple> //for tuple

using namespace std;

int main()
{    
    std::tuple<int, float, char,std::string> t1 
               (5,23.8,'a',"MainFunda");   
              //created tuple holding 4 data values
	
    
    int iData = std::get < 0 > (t1);  
             // get first data member of tuple
     	
    cout <<"Int is "<< iData
         <<endl;
	
     std::get<0>(t1) = 30;              
            // set first data member of tuple
	
     iData = std::get<0>(t1);
	
     cout <<"Int after modification "
          << iData
          <<endl;
    
     return 0;
}

Output

output of code demonstrating get( ) function with tuple

tuple_size( )

At any time we can get the size of tuple by using tuple_size( ) function.

#include <iostream>     // main header
#include <tuple>        // tuple header
#include<string>        // string header
using namespace std;

int main ()
{
     std::tuple<int,char,float,std::string> t1 
           (1,'a',1.1,"MainFunda");
 
     cout << "size of tuple is "
          <<std::tuple_size<decltype(t1)>::value
          << endl;    
              // value contain the number of elements 
 
  return 0;
}

Output

output of code using tuple_size( ) function

make_tuple( )

The function make_tuple( ) is used for creating a tuple from a list of elements passed in the arguments. List of elements which we are passing as argument can be of different data types.

#include <iostream>   //main header
#include <tuple>      // for tuple  

using namespace std; 
 
int main()
{    
    auto t1 = std::make_tuple(1,2.4, "MainFunda");    
             // tuple creation

    cout << "The value of t1 is " 
         << std::get<0>(t1) 
         << ", " 
         << std::get<1>(t1) 
         << ", "
         << std::get<2>(t1) 
         <<endl;
   
}

Output

output of code using make_tuple( ) function

forward_as_tuple( )

The function forward_as_tuple( ) creates a temporary tuple as r-value reference from argument passed as parameter to this function.

// forward_as_tuple example
#include <iostream>     // main header
#include <tuple>        // for tuple

using namespace std;

void show_Data(std::tuple<int&&, int&&> t1) 
{
    std::cout << std::get<0>(t1) 
              << ", " 
              << std::get<1>(t1) 
              << endl;
}

int main() 
{
  show_Data (std::forward_as_tuple(30,30));
 
  return 0;
}

Output

output of code using forward_as_tuple( ) function for forwarding a tuple in c++

tuple_cat( )

The function tuple_cat( ) concatenates two or more tuples which are passed as the arguments.

#include <iostream>  //main header
#include <string>    // for string
#include <tuple>    // for tuple
using namespace std;

int main () {

   std::tuple<char,std::string> t1 ('c',"MainFunda");
   std::pair<int,float> t2 (100,12.5);

   auto t3 = std::tuple_cat ( t1, t2 );

   std::cout << "ConCat T3" <<endl ;
  
   std::cout << std::get<0>(t3) << endl;
   std::cout << std::get<1>(t3) << endl;
   std::cout << std::get<2>(t3) << endl;
   std::cout << std::get<3>(t3) << endl;

   auto t4 = std::tuple_cat(t3, 
                           std::make_tuple(" test", 
                                           "begin"));
   std::cout << "ConCat T4" <<endl ;
   std::cout << std::get<0>(t4) << endl;
   std::cout << std::get<1>(t4) << endl;
   std::cout << std::get<2>(t4) << endl;
   std::cout << std::get<3>(t4) << endl;
   std::cout << std::get<4>(t4) << endl;
   std::cout << std::get<5>(t4) << endl;
  
   return 0;
}

Output

output of code using tuple_cat( ) function

What are Pairs in C++?

Pairs creates class template from 2 different or same datatypes . It is a specialized version of tuple with 2 elements.

Header File required for pairs

#include <utility>

Basic example of Pairs

lets see below simple example for creating and displaying the Pair content

#include <iostream>     // main header
#include <utility>      // for pair

using namespace std;

int main () 
{
   std::pair <int,float> p1 (1,2);
   std::cout << "foo: " 
             << p1.first 
             << ", " 
             << p1.second 
             << endl;

  return 0;
}

Output

basic example of using std::pairs< >

Pair-Helper Functions

make_pair( )

This function creates a pair object from its first and second argument.

#include <iostream>     // main header
#include <utility>      // for pair

using namespace std;

int main () {
   std::pair <int,float> p1 = make_pair(1,2);

   std::cout << "foo: " 
   << p1.first 
   << ", " 
   << p1.second 
   << endl;
  
   return 0;
}

Output

output of basic example with make_pair( ) function

What is a Tie of in C++?

We have read tuple data types above. A tie creates an L-vlaue tuple for future use. Tie is mainly used for extracting element from tuple.

Basic Example with Tie

// packing/unpacking tuples
#include <iostream>     // std::cout
#include <tuple>        // std::tuple, std::make_tuple, std::tie

int main ()
{  
  int i1;
  float f1;
  std::tuple<int,float> t1 = std::make_tuple (1, 22.3);          
                 // packing values into tuple

  std::tie (i1,f1) = t1;   //unpacking tuple into variables

  std::cout << "1st element " 
            << i1 
            << '\n';

  std::cout << "2nd element: " 
            << f1 << '\n';
  
 
  //example to ignore some element
   std::tie (i1, std::ignore) = t1;   
                   // unpacking tuple into variables 
                   //and ignoring 2ns element

   std::cout << "1st element " 
             << i1 
             << '\n';

  return 0;
}

Output

output of code using std::tie< >

Main Funda: Tuples provides additional flexibility to play around with user-defined data types

Related Topics:

Lambda in C++11
Lambda in C++17
What are the drawbacks of using enum ?
Which member functions are generated by compiler in class?
C++ Multithreading: Understanding Threads
How to stop compiler from generating special member functions?
Compiler Generated Destructor is always non-virtual
How to make a class object un-copyable?
Why virtual functions should not be called in constructor & destructor ?
Explaining C++ casts
How std::forward( ) works?
Rule of Three
How std::move() function works?
What is reference collapsing?
How delete keyword can be used to filter polymorphism
emplace_back vs push_back

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