Composite pattern of std::functions - c++

Question

I am trying to implement a composite pattern for std::functions with use of template classes, where each composite class processes the return values of its children.
So the pattern classes might look something like this:
class AbstractClass {
public:
virtual void process() = 0;
};
template<typename ReturnType>
class PrimitiveClass : public AbstractClass {
public:
ReturnType process() {
// please note, that the result is not returned by the return statement
return this->func(); //this is just for simplicity
}
private:
std::function<ReturnType()> func;
}
template<typename ReturnType, typename ...Args>
class CompositeClass : public AbstractClass {
public:
ReturnType process() {
// --> This is where I want to process all children first and then pass their return values to this->func
// the following code is kind of a pseudo code:
for(auto it = vector.begin(); it != vector.end(); ++it {
results.add((**it).process())
}
return this->func(results)
}
private:
std::function<ReturnType(Args...)> func;
std::vector<std::shared_ptr<AbstractClass>> children;
};
So for example, I have a CompositeClass with a std::function<int(int, double, bool) and the argument types of that function are also the ReturnTypes of its children. And I want to pass the return values of the children to above-mentioned std::function
Can anyone think of a way, how I can achieve this?

Answer 1

If I understand what you want (and if I'm not wrong)...
(1) to solve the problem of the no-covariant returned value from process() (see comment from Igor Tandetnik) you need a template abstract class to express the correct return value; by example
template <typename T>
struct abstClass
{ virtual T process() const = 0; };
(2) so your CompositeClass (renamed nodeClass, in my following example) inherit from abstClass<ReturnType>
(3) your PrimitiveClass is useless because you can manage the case (reference to a function without arguments) as a CompositeClass with zero Args
(4) you need a leafClass to handle basic values
(5) in CompositeClass (nodeClass), children, instead of a std::vector of shared_ptr<AbstractClass> (that can't do what do you want), can be a
std::tuple<std::shared_ptr<abstClass<Args>>...> children;
Given these points, I propose the following solution (that, unfortunately, is C++14 because use std::index_sequence and std::make_index_sequence that are available starting from C++14; but if you need a C++11 solution, isn't difficult write substitutes for they)
#include <tuple>
#include <memory>
#include <iostream>
#include <functional>
template <typename T>
struct abstClass
{ virtual T process() const = 0; };
template <typename T>
class leafClass : public abstClass<T>
{
private:
T value;
public:
leafClass (T && v0) : value { std::forward<T>(v0) }
{ }
T process () const
{ return value; };
};
template <typename RetT, typename ... ArgTs>
class nodeClass : public abstClass<RetT>
{
private:
using funcT = std::function<RetT(ArgTs...)>;
template <typename T>
using shrPAC = std::shared_ptr<abstClass<T>>;
funcT func;
std::tuple<shrPAC<ArgTs>...> childrens;
template <std::size_t ... Is>
RetT processH (std::index_sequence<Is...> const &) const
{ return func(std::get<Is>(childrens)->process()...); }
public:
nodeClass (funcT && f0, shrPAC<ArgTs> && ... as)
: func { std::forward<funcT>(f0) },
childrens { std::forward<shrPAC<ArgTs>>(as)... }
{ }
RetT process () const
{ return processH(std::make_index_sequence<sizeof...(ArgTs)>{}); }
};
int main ()
{
auto func0 = [](int i, double d, bool b) { return int( b ? i+d : i-d ); };
auto shpLci = std::make_shared<leafClass<int>>(1);
auto shpLcd = std::make_shared<leafClass<double>>(2.2);
auto shpNb = std::make_shared<nodeClass<bool>>([](){ return true; });
auto shpNc0 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpLci, shpLcd, shpNb);
auto shpNc1 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpNc0, shpLcd, shpNb);
auto shpNc2 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpNc1, shpLcd, shpNb);
std::cout << shpNc0->process() << std::endl; // print 3
std::cout << shpNc1->process() << std::endl; // print 5
std::cout << shpNc2->process() << std::endl; // print 7
}