Further Options

Let’s look at a couple more things you can do using function. First, we don’t actually have to declare six function<double(double)> objects in Listing 18.8. Instead, we can use a temporary function<double(double)> object as an argument to the use_f() function:

typedef function<double(double)> fdd; // simplify the type declaration
cout << use_f(y, fdd(dub)) << endl;   // create and initialize object to dub
cout << use_f(y, fdd(square)) << endl;
...

Second, Listing 18.8 adapts the second arguments in use_f() to match the formal parameter f. Another approach is to adapt the type of the formal parameter f to match the original arguments. This can be done by using a function wrapper object as the second parameter for the use_f() template definition. We can define use_f() this way:

#include <functional>
template <typename T>
T use_f(T v,  std::function<T(T)> f)   // f call signature is T(T)
{
    static int count = 0;
    count++;
    std::cout << "  use_f count = " << count
              << ", &count = " << &count << std::endl;
    return f(v);
}

Then the function calls can look like this:

cout << "  " << use_f<double>(y, dub) << endl;
...
cout << "  " << use_f<double>(y, Fp(5.0)) << endl;
...
cout << "  " << use_f<double>(y, [](double u) {return u*u;}) << endl;

The arguments dub, Fp(5.0), etc., are not themselves type function<double(double)>, and so the calls use <double> after use_f to indicate the desired specialization. Thus T is set to double, and std::function<T(T)> becomes std::function<double(double)>.

Variadic Templates

Variadic templates provide a means to create template functions and template classes that accept a variable number of arguments. We’ll look at variadic template functions here. For example, suppose we want a function that will accept any number of parameters of any type, providing the type can be displayed with cout, and display the arguments as a comma-separated list. For instance, consider this code:

int n = 14;
double x = 2.71828;
std::string mr = "Mr. String objects!";
show_list(n, x);
show_list(x*x, '!', 7, mr);

The goal is to be able to define show_list() in such a way that this code would compile and lead to this output:

14, 2.71828
7.38905, !, 7, Mr. String objects!

There are a few key points to understand in order to create variadic templates:

• Template parameter packs

• Function parameter packs

• Unpacking a pack

• Recursion