Cached or precalculated immutable functions in C # / C ++

Question

By “immutable function” or “immutable method” I mean a function whose result will never change if you give it the same arguments.

I would be interested to know if anyone knows a more general or less detailed solution if you want to cache the pre-computed value (s) of an immutable function.

Let me explain what I mean with a simple example:

//Let assume that ComputeStuff() returns a widely used value 
//and that 
//1. It is immutable (it will always return the same result)
//2. its performance is critical, and it cannot be accepted to compute
//   the result at each call, because the computation is too slow
//I show here a way to solve the problem, based on a cached result.
//(this example works in a case of a method with no arguments. 
// A hash would be required in order to store multiple precomputed results 
//depending upon the arguments)
private string mComputeStuff_Cached = null;
public string ComputeStuff()
{
  if (mComputeStuff_Cached != null)
    return mComputeStuff_Cached ;

  string result;
  //
  // ...
  //Do lots of cpu intensive computation in order to compute "result" 
  //or whatever you want to compute
  //(for example the hash of a long file)
  //...
  //

  mComputeStuff_Cached  = result;
  return mComputeStuff_Cached ;
}

:
- ++ ++, - " " , "" " " ( ).

Answer 1

" Memoization" . memoization ( , boost , ). - "memoize" "memoization" .

Wikibooks: ++/ /

Answer 2

, , Func<T>, , /, - # "":

using System;
static class Program {
    static void Main() {
        var func = CachedFunc.Create(() => int.Parse(Console.ReadLine()));

        Console.WriteLine(func.Value);
        Console.WriteLine(func.Value);
    }
}
static class CachedFunc {
    public static CachedFunc<T> Create<T>(Func<T> func) {
        return new CachedFunc<T>(func);
    }
}
class CachedFunc<T> {
    T value;
    Func<T> func;
    public CachedFunc(Func<T> func){
        if (func == null) throw new ArgumentNullException("func");
        this.func = func;
    }
    public T Value {
        get {
            if (func != null) {
                value = func();
                func = null;
            }
            return value;
        }
    }
    public static explicit operator T(CachedFunc<T> func) {
        return func.Value; }
}

Answer 3

- :

#include <functional>
#include <type_traits>
#include <map>
#include <tuple>

//requires c++14
auto add_function_cache = [](auto fun) {
    using fun_type = decltype(fun);
    return ([=](auto... run_args){
        using fun_return_type = std::result_of_t<fun_type(decltype(run_args)...)>;
        static std::map<
            std::tuple<decltype(run_args)...>,
            fun_return_type
        > result_cache;
        std::tuple<decltype(run_args)...> tuple(run_args...);
        if (result_cache.find(tuple) == result_cache.end()) {
            fun_return_type rv = fun(run_args...);
            result_cache[tuple] = rv; 
            return rv; 
        }   
        else {
            return result_cache[tuple];
        }   
    }); 
};

template <typename R, class... Args> auto
add_function_cache_old(std::function<R(Args...)> fun)
-> std::function<R(Args...)>
{
    std::map<std::tuple<Args...>, R> cache;
    return [=](Args... args) mutable  {
        std::tuple<Args...> t(args...);
        if (cache.find(t) == cache.end()) {
            R rv = fun(args...);
            cache[t] = rv; 
            return rv; 
        }   
        else {
            return cache[t];
        }   
    };  
};

:

//function_cache - usage
auto fib_cache = add_function_cache(&fib);

//function_cache_old - usage
function<decltype(fib)> fib_fn = &fib;
auto fib_cache_old = add_function_cache_old(fib_fn);

fib_cache(10);
fib_cache(10);

, , (fun) . (run_args) . , , , , (run_args) (). , . , .

Jan

Answer 4

++ , , const () mutable :

class Computer {
    mutable string cache;
  public:
    // I wouldn't call it ComputeXXX
    // since I want to hide the implementation
    // details from my client: for the client
    // there is no change in state due to a call
    // to this function
    const string& StringVal() const {
         if (cache.empty()) {
                // compute cache
         }
         return cache;
    }
    // ...
};              

Answer 5

:

private string mComputeStuff_Cached = null;
public string ComputeStuff()
{
  if (mComputeStuff_Cached == null) {
    string result;
    //
    // ...
    //Do lots of cpu intensive computation in order to compute "result" 
    //or whatever you want to compute
    //(for example the hash of a long file)
    //...
    //

    mComputeStuff_Cached  = result;
  }

  return mComputeStuff_Cached ;
}

:

• ++ const, , const . , (), mutable.
• " " " ". - const, "semantic const" ( , ), "bitwise const".
• const , , . "" , "" . , , , .

Answer 6

- ( ).

const member . , .

Answer 7

++

class CClass
{

  private:
     std::string* mComputeStuff_Cached;

  public:
     CClass()
       mComputeStuff_Cached(NULL)
     {

     }

     ~CClass()
     {
           delete mComputeStuff_Cached;
     }


     std::string ComputeStuff()
     {
         if (mComputeStuff_Cached != NULL)
         {
             return mComputeStuff_Cached
         }
         else
         {
             std::string calcedAnswer;
             ...
             // store away answer
             mComputeStuff_Cached = new std::string(calcedAnswer);
         }
     }
};

I'm not sure if checking is enough if mComputeStuff_Cached () is enough. It may be that empty () is a legitimately cached result.

Answer 8

You can use the keyword staticinside the function. It will be calculated only once:

std::string GetWidelyUsedValue()
{
   static std::string value = ComputeStuff() ;
   return value ;
}

std::string ComputeStuff()
{
   // Compute the string here.
}