Template and Function Parameter Packs

As a starting point to see how parameter packs work, let’s consider a simple template function, one that displays a list consisting of just one item:

template<typename T>
void show_list0(T value)
{
    std::cout << value << ", ";
}

This definition has two parameter lists. The template parameter list is just T. The function parameter list is just value. A function call such as the following sets T in the template parameter list to double and value in the function parameter list to 2.15:

show_list0(2.15);

C++11 provides an ellipsis meta-operator that enables you to declare an identifier for a template parameter pack, essentially a list of types. Similarly, it lets you declare an identifier for a function parameter pack, essentially a list of values. The syntax looks like this:

template<typename... Args>    // Args is a template parameter pack
void show_list1(Args... args) // args is a function parameter pack
{
...
}

Args is a template parameter pack, and args is a function parameter pack. (As with other parameter names, any name satisfying C++ identifier rules can be used for these packs.) The difference between Args and T is that T matches a single type, whereas Args matches any number of types, including none. Consider the following function call:

show_list1('S', 80, "sweet", 4.5);

In this case the parameter pack Args contains the types matching the parameters in the function call: char, int, const char *, and double.

Next, much as

void show_list0(T value)

states that value is of type T, the line

void show_list1(Args... args)  // args is a function parameter pack

states that args is of type Args. More precisely, this means that the function pack args contains a list of values that matches the list of types in the template pack Args, both in type and in number. In this case, args contains the values 'S', 80, "sweet", and 4.5.

In this manner, the show_list1() variadic template can match any of the following function calls:

show_list1();
show_list1(99);
show_list1(88.5, "cat");
show_list1(2,4,6,8, "who do we", std::string("appreciate));

In the last case, the Args template parameter pack would contain the types int, int, int, int, const char *, and std::string, and the args function parameter pack would contain the matching values 2, 4, 6, 8, "who do we", and std::string("appreciate").