Why does template argument deduction/substitution fail here?

Question

I am trying to write a simple template that I can use for memoization with functions taking a single argument:

#include <map>          

template <typename F,typename OUT,typename IN> 
OUT memoization(IN in){
    static std::map<IN,OUT> memo;
    static typename std::map<IN,OUT>::iterator found = memo.find(in);
    if (found != memo.end()) { return found->second; }  
    OUT res = F(in);
    memo(in) = res;
    return res;
}

double test(double x) { return x*x; }

int main(){
    for (int i=0;i<5;i++){
        memoization<test,double,double>(i*0.5);
    }
}

But i get the error:

error: no matching function for call to 'memoization(double)'

note: candidate is:

note: template OUT memoization(IN)

note: template argument deduction/substitution failed:

Why does this fail to compile?

Actually I dont understand why template argument deduction/substitution is taking place at all when I specify all the template parameters.

I am using gcc version 4.7.2 (no C++11 enabled)

PS: the template has many more errors than I first realized, but I leave it as is...

Answer 1

Your function template takes three type arguments:

template <typename F,typename OUT,typename IN> 
OUT memoization(IN in) { ... }

You're passing in test for F . test isn't a type, it's a value. Also, the expression F(in) in your function template is wrong for the same reason.

---

This approach in general is pretty flawed as it seems pretty backwards from what actually is going on. Namely, it's the function that's being memoized, not a value. Also requiring the function value at compile time is pretty limiting.

A better approach is to treat memoization as a decorator. That is:

template <class F>
Memoized<F> memoize(F f) {
    return {f};
}

such that:

auto memo_test = memoize(test);
memo_test(0); // performs computation
memo_test(0); // doesn't perform computation
memo_test(0); // ditto

I leave the implementation of Memoized<T> as an exercise.

Answer 2

Why does template argument deduction/substitution fail here?

a. Because there are 3 template arguments and only one actual argument, so two of them are undeduce-able (is that a word?).

b. There is a syntax error. Template argument F is a type, not a callable object.

If this has to work in a pre-c++11 environment, boost 's result_of can help:

#include <map>         
#include <boost/utility/result_of.hpp>

//
// now that template arguments are all used when collecting function
// arguments, the types F and IN can be deduced.
//    
template <typename F,typename IN> 
typename boost::result_of<F(IN)>::type memoization(F f, IN in)
{
  typedef typename boost::result_of<F(IN)>::type OUT;
    static std::map<IN,OUT> memo;
    static typename std::map<IN,OUT>::iterator found = memo.find(in);
    if (found != memo.end()) { return found->second; }  
    OUT res = f(in);
    memo[in] = res;
    return res;
}

double test(double x) { return x*x; }

int main(){
    for (int i=0;i<5;i++){
        memoization(test, i*0.5);
    }
}

Answer 3

The answer already got a satisfactory answers, however I was curious if I could get it running with my approach (and with purely pre C++11). Actually it is possible to pass a function pointer as template parameter, one just has to specify this on the template parameter instead of letting it expect a type parameter:

#include <iostream>
#include <map>
using namespace std;

template <class T, class R, R (*Func)(T)>
R memoized(T in) {
    static std::map<T,R> memo;
    typename std::map<T,R>::iterator found = memo.find(in);
    if (found != memo.end()) { return found->second; }  
    std::cout << "not found" << std::endl;
    R res = Func(in);
    memo[in] = res;
    return res;
}

double test(double x){return x*x;}
double test2(double x){return x;}

int main() {
    std::cout << memoized<double,double,test>(1) << std::endl;
    std::cout << memoized<double,double,test>(1) << std::endl;
    std::cout << memoized<double,double,test>(1) << std::endl;
    std::cout << std::endl;
    std::cout << memoized<double,double,test2>(1) << std::endl;
    std::cout << memoized<double,double,test2>(1) << std::endl;
    std::cout << memoized<double,double,test2>(1) << std::endl;

    return 0;
}

output:

not found
1
1
1

not found
1
1
1

Still not sure if this is a good approach, but it seems to work.