Functional Decorators (C++)

functional_decorator.cpp

#include <iostream>
#include <sstream>
#include <string>
#include <functional>

struct Logger
{
  Logger( const std::function<void(void)>& func, std::string name )
  : func { func }, name { name } {}

  void operator()()
  {
    std::cout << "Entering Function " << name << std::endl;
    func();
    std::cout << "Exiting " << name << std::endl;
  }

private:
  std::function<void(void)> func;
  std::string name;
};

template <typename Func>
struct GenericLogger
{
  GenericLogger( const Func& func, std::string name )
  : func { func }, name { name } {}

  void operator()()
  {
    std::cout << "Entering via GenericLogger Function " << name << std::endl;
    func();
    std::cout << "Exiting " << name << std::endl;
  }

private:
  Func func;
  std::string name;
};

template <typename> struct FunctionalLogger;

template <typename R, typename ...Args>
struct FunctionalLogger<R(Args...)>
{
  FunctionalLogger( const std::function<R(Args...)>& func, std::string name )
  : func { func }, name { name } {}

  R operator()( Args ...args )
  {
    std::cout << "Entering via FunctionalLogger Function " << name << std::endl;
    auto result = func( std::forward<Args>(args)... );
    std::cout << "Exiting " << name << std::endl;
    return result;
  }
private:
  std::function<R(Args...)> func;
  std::string name;
};

template <typename R, typename ...Args>
FunctionalLogger<R(Args...)> makeFuncLogger(
  R (*func)( Args... ), std::string name )
{
  return FunctionalLogger<R(Args...)>(
    std::function<R(Args...)>( func ), name
  );
}

int add2Numbers( int x, int y )
{
  std::cout << "x=" << x << " y=" << y << std::endl;
  return x + y;
}

int main()
{
  auto loggedHello = GenericLogger(
    [](){ std::cout << "Hello World" << std::endl; },
    "SayHello()"
  );
  loggedHello();

  auto withReturnDouble = FunctionalLogger<int(int)>(
    []( int x ) -> int { std::cout << "x=" << x << std::endl; return x*2; },
    "double()"
  );
  std::cout << withReturnDouble(4) << std::endl;


  auto funcLogger = makeFuncLogger( add2Numbers, "add2Numbers()");
  std::cout << funcLogger(5, 10) << std::endl;

  return 0;
}