Variadic Templates example

Question

Consider following code, I don\'t understand why empty function of print must be defined.

#include 
using namespace std;

void print()
{   
}   
         


        

Answer 1

CORRECT WAY:

Variadic templates is strictly related to induction, a mathematical concept.

The compiler resolves the following function call

print('a', 3, 4.0f);

into

std::cout<< 'a' <

which is resolved into

std::cout<< 'a' <

which is resolved into

std::cout<< 'a' <

At this point it searches for a function overload whose match is the empty function.

• All functions that have 1 or more arguments are matched to the variadic template
• All functions that have no argument are matched to the empty function

The culprit is that you must have, for every possible combination of parameters, only 1 function.

---

ERROR 1:

Doing the following would be an error

template< typename T>
void print( const T& arg) // first version
{   
    cout<< arg<
void print (const T& firstArg, const Types&... args) // second version
{   
    cout << firstArg << endl; // print first argument
    print(args...); // call print() for remaining arguments
}

Because when you call print the compiler doesn't know which function to call.

Does print(3) refers to "first" or "second" version? Both would be valid because the first has 1 parameter, and the second can accept one parameter too.

print(3); // error, ambiguous, which one you want to call, the 1st or the 2nd?

---

ERROR 2:

The following would be an error anyway

// No empty function

template 
void print (const T& firstArg, const Types&... args) 
{   
    cout << firstArg << endl; // print first argument
    print(args...); // call print() for remaining arguments
}

In fact, if you use it alone without the compiler would do

 print('k', 0, 6.5);

which is resolved into

 std::cout<<'k'<

which is resolved into

 std::cout<<'k'<

which is resolved into

 std::cout<<'k'<

As you see in the last attempt, the compiler tries to call print() with no arguments. However if such a function does not exists it is not called, and that's why you should provide that empty function (don't worry, the compiler will optimize code so empty functions don't decrease performance).