Variadic function templates in C++

Variadic templates are class or function templates, that can take any variable(zero or more) number of arguments. In C++, templates can have a fixed number of parameters only that have to be specified at the time of declaration. However, variadic templates help to overcome this issue. Douglas Gregor and Jaakko Järvi came up with this feature for C++.

Variadic arguments are very similar to arrays in C++. We can easily iterate through the arguments, find the size(length) of the template, can access the values by an index, and can slice the templates too. 

So basically, Variadic function templates are functions that can take multiple numbers of arguments.

Syntax:

template(typename arg, typename... args)
return_type function_name(arg var1, args... var2)

Note: The arguments must be put inside angular brackets. 

Below is an example in C++ to show how we can use a variadic function template:

CPP

// C++ program to demonstrate working of
// Variadic function Template
#include <iostream>
using namespace std;

// To handle base case of below recursive
// Variadic function Template
void print()
{
    cout << "I am empty function and "
            "I am called at last.\n";
}

// Variadic function Template that takes
// variable number of arguments and prints
// all of them.
template <typename T, typename... Types>
void print(T var1, Types... var2)
{
    cout << var1 << endl;

    print(var2...);
}

// Driver code
int main()
{
    print(1, 2, 3.14,
          "Pass me any "
          "number of arguments",
          "I will print\n");

    return 0;
}

Output

1
2
3.14
Pass me any number of arguments
I will print

I am empty function and I am called at last.

Remember that templates are replaced by actual functions by the compiler.

Explanation: The variadic templates work as follows : 

The statement, print(1, 2, 3.14, “Pass me any number of arguments”, “I will print\n”); is evaluated in the following manner: 

Firstly, the compiler resolves the statement into 

cout<< 1 <<endl ;
print(2, 3.14, "Pass me any number of arguments", 
      "I will print\n");

Now, the compiler finds a print() function which can take those arguments and in result executes the variadic print() function again in a similar manner: 

cout<< 2 <<endl ;
print(3.14, "Pass me any number of arguments", 
      "I will print\n");

Again, it is resolved into the following forms : 

cout<< 3.14 <<endl ;
print("Pass me any number of arguments", 
      "I will print\n");

cout<< "Pass me any number of arguments" <<endl ;
print("I will print\n");

cout<< "I will print\n" <<endl ;
print();

Now, at this point, the compiler searches for a function overload whose match is the empty function i.e. the function which has no argument. This means that all functions that have 1 or more arguments are matched to the variadic template and all functions that with no argument are matched to the empty function.

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