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.