How Can Template Functions be Passed as Arguments in C ?

Template Function as a Template Argument Explained

In C , generic programming can be achieved using function pointers or templates. Templates offer efficiency as they allow the compiler to inline functions, leading to better performance.

Consider a scenario where two functions, a and b, perform similar operations but with slight variations. To avoid code duplication, a generic function can be defined using templates:

<code class="c++">template<void (*param)(int)>
void function() {
    param(123);
    param(456);
}</code>

This template takes a function pointer as an argument, allowing for runtime selection of the specific function to be executed. In the example, function<> can be called with either a or b as the argument.

Template Functions with Template Arguments

However, limitations arise when a and b are themselves generic functions. Template parameters can be types, template types, or values, none of which directly support passing template functions as arguments.

To overcome this challenge, one can employ "template template parameters." However, using a template function as a type requires instantiation, making it impractical. A workaround involves using dummy structures:

<code class="c++">struct a { static void foo () {}; };
struct b { static void foo () {}; };

template <template <typename P> class T>
void function ()
{
    T<SomeObj>::foo ();
    T<SomeOtherObj>::foo ();
}

int main ()
{
    function<a>();
    function<b>();
}</code>

In this example, a and b are encapsulated as static member functions within structs. function takes a template struct as an argument, essentially representing the template function to be invoked. By passing this struct, the specific foo function can be executed for different types.

Therefore, this approach allows for defining generic functions that can accept template functions as arguments, enabling flexible and reusable code in C generics.

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