Replacing switch statements when interfacing between templated and non-templated code

Question

The X:

A common pattern I'm seeing is that the underlying code for a function is templates, but for "reasons" the template code is not available at the upper layer (pick from aversion to templates in interface, the need for a shared library and not to expose implementation to customer, reading type settings at run time instead of compile time, etc.).

This often makes the following:

struct foo { virtual void foo() = 0;}
template <typename T> struct bar : public foo
{
    bar( /* Could be lots here */);
    virtual void foo() { /* Something complicated, but type specific */}
};

And then an initialize call:

foo* make_foo(int typed_param, /* More parameters */)
{
    switch(typed_param)
    {
        case 1: return new bar<int>(/* More parameters */);
        case 2: return new bar<float>(/* More parameters */);
        case 3: return new bar<double>(/* More parameters */);
        case 4: return new bar<uint8_t>(/* More parameters */);
        default: return NULL;
    }
}

This is annoying, repetitive, and error prone code.

So I says to myself, self says I, there has GOT to be a better way.

The Y:

I made this. Do you all have a better way?

////////////////////////////////////
//////Code to reuse all over the place
///
template <typename T, T VAL>
struct value_container
{
    static constexpr T value() {return VAL;}
};

template <typename J, J VAL, typename... Ts>
struct type_value_pair
{
    static constexpr J value() {return VAL;}

    template <class FOO>
    static auto do_things(const FOO& foo)->decltype(foo.template do_things<Ts...>()) const
    {
        foo.template do_things<Ts...>();
    }
};

template <typename T>
struct error_select
{
    T operator()() const { throw std::out_of_range("no match");}
};

template <typename T>
struct default_select
{
    T operator()() const { return T();}
};

template <typename S, typename... selectors>
struct type_selector
{
    template <typename K, class FOO, typename NOMATCH, typename J=decltype(S::do_things(FOO()))>
    static constexpr J select(const K& val, const FOO& foo=FOO(), const NOMATCH& op=NOMATCH())
    {
        return S::value()==val ? S::do_things(foo) : type_selector<selectors...>::template select<K, FOO, NOMATCH, J>(val, foo, op);
    }
};

template <typename S>
struct type_selector<S>
{
    template <typename K, class FOO, typename NOMATCH, typename J>
    static constexpr J select(const K& val, const FOO& foo=FOO(), const NOMATCH& op=NOMATCH())
    {
        return S::value()==val ? S::do_things(foo) : op();
    }
};

////////////////////////////////////
////// Specific implementation code
class base{public: virtual void foo() = 0;};

template <typename x>
struct derived : public base
{
    virtual void foo() {std::cout << "Ima " << typeid(x).name() << std::endl;}
};


struct my_op
{
    template<typename T>
    base* do_things() const
    {
        base* ret = new derived<T>();
        ret->foo();
        return ret;
    }
};

int main(int argc, char** argv)
{
    while (true)
    {
        std::cout << "Press a,b, or c" << std::endl;
        char key;
        std::cin >> key;

        base* value = type_selector<
            type_value_pair<char, 'a', int>,
            type_value_pair<char, 'b', long int>,
            type_value_pair<char, 'c', double> >::select(key, my_op(), default_select<base*>());

        std::cout << (void*)value << std::endl;
    }

    /* I am putting this in here for reference. It does the same
       thing, but the old way: */

    /*
        switch(key)
        {
            case 'a':
              {
                  base* ret = new derived<int>();
                  ret->foo();
                  value = ret;
                  break;
              }

            case 'b':
              {
                  base* ret = new derived<char>();
                  ret->foo();
                  value = ret;
                  break;
              }

            case 'c':
              {
                  base* ret = new derived<double>();
                  ret->foo();
                  value = ret;
                  break;
              }

            default:
                return NULL;
        }
    */
}

Problems I see with my implementation:

1. It is clear and readable as mud
2. Template parameters MUST be types, have to wrap values in types (template <typename T, T VAL> struct value_container { static constexpr T value() {return VAL;} };)
3. Currently no checking/forcing that the selectors are all type-value pairs.

And the only pros:

1. Removes code duplication.
2. If the case statement gets high/the contents of do_things gets high, then we can be a little shorter.

Has anyone do something similar or have a better way?

Answer 1

You can always walk a type list indexed by type_param, as in:

struct foo 
{
    virtual ~foo() = default;
    /* ... */
};

template<typename T>
struct bar : foo 
{ /* ... */ };


template<typename TL> 
struct foo_maker;

template<template<typename...> class TL, typename T, typename... Ts> 
struct foo_maker<TL<T, Ts...>>
{
    template<typename... Us>
    std::unique_ptr<foo> operator()(int i, Us&&... us) const
    {
        return i == 1 ?
            std::unique_ptr<foo>(new bar<T>(std::forward<Us>(us)...)) :
            foo_maker<TL<Ts...>>()(i - 1, std::forward<Us>(us)...); }
};

template<template<typename...> class TL> 
struct foo_maker<TL<>>
{
    template<typename... Us>
    std::unique_ptr<foo> operator()(int, Us&&...) const
    { return nullptr; }
};


template<typename...>
struct types;


template<typename... Us>
std::unique_ptr<foo> make_foo(int typed_param, Us&& us...)
{ return foo_maker<types<int, float, double, uint8_t>>()(typed_param, std::forward<Us>(us)...); };

Note: this factory function is O(n) (although a clever compiler could make it O(1)), while the switch statement version is O(1).

Answer 2

Just to expand YoungJohn's comment, it looks like this (I've included a single initialization of the operator, and it could be made simpler if there was no parameters, but if there are no parameters there is little reason to do this anyway :-P).

#include <functional>
#include <map>


////////////////////////////////////
//////specific impmenetation code
class base{public: virtual void foo() = 0;};

template <typename x>
struct derived : public base
{
    virtual void foo() {std::cout << "Ima " << typeid(x).name() << std::endl;}
};

struct my_op
{
    int some_param_; /// <shared parameter

    my_op(int some_param) : some_param_(some_param){} /// <constructor

    template<typename T>
    base* do_stuff() const
    {
        std::cout << "Use some parameter: " << some_param_ << std::endl;
        base* ret = new derived<T>();
        ret->foo();
        return ret;
    }
};

base* init_from_params(int some_param, char key)
{
    my_op op(some_param);
    using factoryFunction = std::function<base*()>;
    std::map<char, factoryFunction> mp
    {
        { 'a', std::bind(&my_op::do_stuff<int>, &op)},
        { 'b', std::bind(&my_op::do_stuff<long int>, &op)},
        { 'c', std::bind(&my_op::do_stuff<double>, &op)}
    } ;
    factoryFunction& f = mp[key];
    if (f)
    {
        return f();
    }
    return NULL;
}


int main(int argc, char** argv)
{
    volatile int parameters = 10;

    while (true)
    {
        std::cout << "Press a, b, or c" << std::endl;
        char key;
        std::cin >> key;

        base* value = init_from_params(parameters, key);

        std::cout << (void*)value << std::endl;
    }
}

Pros: so much shorter, so much more standard, so much less weird template stuff. It also doesn't require the templated arguments to all be types, we can select whatever we want to initialize the function.

Cons: In theory, it could have more overhead. In practice, I totally doubt that the overhead would ever matter.

I like it!

Answer 3

template<class T>
foo* make_foo(int typed_param,/*more params*/)
{
    return new bar<T>(/*more params*/);
}