I have the following code where I implement dispatching on runtime value to interpret the data in certain way(in this toy example data can be either uint8_t or short).
Code seems to work, but I am wondering if I can somehow microoptimize the code so that when I have a hit(processing function matches) processing is stopped (currently even if first element of tuple is a "handler" entire tuple is iterated over at runtime).
#include <boost/mp11/tuple.hpp>
#include <iostream>
uint8_t data[4] = {0,1,100,2};
template<int runtimeId, typename T>
struct kindToType{
static constexpr int id = runtimeId;
using type = T;
};
const auto print =[]<typename T> (const T* data){
if constexpr(std::is_same_v<short, std::remove_cvref_t<T>>){
const short* values = (const short*)data;
std::cout << values[0] << " " << values[1] << std::endl;
} else if constexpr(std::is_same_v<uint8_t, std::remove_cvref_t<T>>){
const uint8_t* values = (const uint8_t*)data;
std::cout << (int)values[0] << " " << (int)values[1]<< " " << (int)values[2] << " " << (int)values[3] << std::endl;;
}
};
static constexpr std::tuple<kindToType<10, uint8_t>, kindToType<11, short>> mappings{};
void dispatch(int kind){
boost::mp11::tuple_for_each(mappings, [kind]<typename Mapping>(const Mapping&) {
if (Mapping::id == kind)
{
print((typename Mapping::type*)data);
}
});
}
int main()
{
// no guarantee that kind is index like(e.g. for two values
// it can have values 47 and 1701)
dispatch(10);
dispatch(11);
}
Notes:
std::function
)break;
.The runtime performance of this heavily depends on the size of your tuple. You can make your own for_each_tuple
implementation that does an early out when your function gets executed:
template<typename FuncTuple, typename Selector>
void tuple_for_each(FuncTuple const& funcTuple, Selector selector)
{
std::apply([selector](auto const& ...funcs)
{
(void)(selector(funcs) || ...);
}, funcTuple);
}
your dispatch would then look like this:
void dispatch(int kind)
{
tuple_for_each(mappings, [kind]<typename Mapping>(const Mapping&)
{
std::cout << "loop, ";
if (Mapping::id == kind)
{
print((typename Mapping::type*)data);
return true;
}
return false;
});
}
If you get rid of the template in your lambda and use auto
instead this code will compile with C++17. We use operator short circuiting to our advantage so the compiler will provide an early out for us. Here is the full code.
Also, note that the cast (const short*)data
is UB.