Functional programming in C++. Implementing f(a)(b)(c)

Question

I have been getting into the basics of functional programming with C++. I am trying to make a function f(a)(b)(c) that will return a + b + c. I successfully implemented the function f(a)(b) which returns a + b. Here is the code for it:

std::function<double(double)> plus2(double a){     return[a](double b){return a + b; }; } 

I just cannot figure out how to implement the function f(a)(b)(c) which as I previously stated should return a + b + c.

Answer 1

You can do it by having your function f return a functor, i.e., an object that implements operator(). Here is one way to do it:

struct sum  {     double val;      sum(double a) : val(a) {}      sum operator()(double a) { return val + a; }      operator double() const { return val; } };  sum f(double a) {     return a; } 

Example

Link

int main() {     std::cout << f(1)(2)(3)(4) << std::endl; } 

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Template version

You can even write a templated version that will let the compiler deduce the type. Try it here.

template <class T> struct sum  {     T val;      sum(T a) : val(a) {}      template <class T2>     auto operator()(T2 a) -> sum<decltype(val + a)> { return val + a; }      operator T() const { return val; } };  template <class T> sum<T> f(T a) {     return a; } 

Example

In this example, T will ultimately resolve to double:

std::cout << f(1)(2.5)(3.1f)(4) << std::endl;