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Variadic template function - Part 1

 In C, we know we have something called printf() function. This printf() function can take any number of arguments of any built-in type (not user-defined type). As an example, the following piece of code is valid for the standard built-in printf() function.

<Code>

const char* msg = "%s can accept %i parameters (or %s).";
printf(msg, std::string("Variadic templates"), 100, "more");

</Code>

The problem in this code is, it will not print the string part, but it gets compiled with the MSVC as well as GCC 8.2 compiler.

However, if I change the code like below, now it prints everything properly.

<Code>

const char* msg = "%s can accept %i parameters (or %s).";
printf(msg, std::string("Variadic templates").c_str(), 100, "more");

</Code>

The printf is like a variadic template function but it sticks to only built-in types. Now the question comes how can I write variadic template functions in C++. In C++ 11 the support has been added.

Let's start with a simple objective, which is to create a function that takes any arbitrary number of parameters of any arbitrary type and just prints them on the screen. As an example, the function shall look like below: 

PrintAllTypes(1, 'C', "Hello World", 0.67, 1.002);

Here goes the code for PrintAllTypes:

<Code>
void PrintAllType() { std::cout << "\n"; } /* This is the base case */

template<typename T, typename... Rest>
void PrintAllType(const T& current, const Rest&... rest) {
    std::cout << current << " ";
    PrintAllType(rest...); // This is a recursive call using pack expansion syntax
}
</Code>

Below is another variant of Print that takes any number of parameters but is limited to numbers only.

<Code>
void Print() { std::cout << "\n"; } /* This is the base case */

template <typename T, 
    typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr, typename... Rest>
void Print(const T& current, const Rest&... rest) {
    std::cout << current << " ";
    Print(rest...); // Again, this is a recursive call using pack expansion syntax
}
</Code>

Let's try the addition of numbers via the variadic template function:

<Code>
template <typename T>
auto AddGeneric(T& t)
{
    return t;
}

template<typename T, typename... Rest, 
    typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr>
auto AddGeneric(T& current, Rest&... rest)
{
    return current + AddGeneric(rest...);
}

// The Client code:
    int x = 10, y = 20; double dbl = 5.5;
    auto res = AddGeneric(x, dbl, y);
    PrintAllType("Add Result: ", res);
</Code>

The output: 
Add Result: 35.5

What if I have a number array and I need to use that array as an argument in a variadic function. One possibility could be using a functor to unpack the array and then use it for a variadic function. 

<Code>
template <typename T>
auto AddGeneric(T& t)
{
    return t;
}

template<typename T, typename... Rest, 
    typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr>
auto AddGeneric(T& current, Rest&... rest)
{
    return current + AddGeneric(rest...);
}

/* Array passed to a variadic function. With the help of a functor unpacking the array */
template<typename Functor, typename T, std::size_t N>
auto AdditionByArray(Functor fn, T(&t)[N])
{
    return AdditionByArrayImpl(fn, t, std::make_index_sequence<N>{});
}

template<typename F, typename T, std::size_t N, std::size_t... idx>
auto AdditionByArrayImpl(F fn, T(&t)[N], std::index_sequence<idx... >)
{
    return fn(t[idx]...);
}

struct AddSupli
{
    template <typename... Arguments>
    auto operator()(Arguments... args)
    {
        return AddGeneric(args...); // This is same recursive call
    }
};
</Code>

So far so good. Will come back to this topic later.
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