Apply function from std::tuple [duplicate] - c++

Question

I'm trying to store in a std::tuple a varying number of values, which will later be used as arguments for a call to a function pointer which matches the stored types.
I've created a simplified example showing the problem I'm struggling to solve:
#include <iostream>
#include <tuple>
void f(int a, double b, void* c) {
std::cout << a << ":" << b << ":" << c << std::endl;
}
template <typename ...Args>
struct save_it_for_later {
std::tuple<Args...> params;
void (*func)(Args...);
void delayed_dispatch() {
// How can I "unpack" params to call func?
func(std::get<0>(params), std::get<1>(params), std::get<2>(params));
// But I *really* don't want to write 20 versions of dispatch so I'd rather
// write something like:
func(params...); // Not legal
}
};
int main() {
int a=666;
double b = -1.234;
void *c = NULL;
save_it_for_later<int,double,void*> saved = {
std::tuple<int,double,void*>(a,b,c), f};
saved.delayed_dispatch();
}
Normally for problems involving std::tuple or variadic templates I'd write another template like template <typename Head, typename ...Tail> to recursively evaluate all of the types one by one, but I can't see a way of doing that for dispatching a function call.
The real motivation for this is somewhat more complex and it's mostly just a learning exercise anyway. You can assume that I'm handed the tuple by contract from another interface, so can't be changed but that the desire to unpack it into a function call is mine. This rules out using std::bind as a cheap way to sidestep the underlying problem.
What's a clean way of dispatching the call using the std::tuple, or an alternative better way of achieving the same net result of storing/forwarding some values and a function pointer until an arbitrary future point?

Answer 1

You need to build a parameter pack of numbers and unpack them
template<int ...>
struct seq { };
template<int N, int ...S>
struct gens : gens<N-1, N-1, S...> { };
template<int ...S>
struct gens<0, S...> {
typedef seq<S...> type;
};
// ...
void delayed_dispatch() {
callFunc(typename gens<sizeof...(Args)>::type());
}
template<int ...S>
void callFunc(seq<S...>) {
func(std::get<S>(params) ...);
}
// ...

Answer 2

The C++17 solution is simply to use std::apply:
auto f = [](int a, double b, std::string c) { std::cout<<a<<" "<<b<<" "<<c<< std::endl; };
auto params = std::make_tuple(1,2.0,"Hello");
std::apply(f, params);
Just felt that should be stated once in an answer in this thread (after it already appeared in one of the comments).
The basic C++14 solution is still missing in this thread. EDIT: No, it's actually there in the answer of Walter.
This function is given:
void f(int a, double b, void* c)
{
std::cout << a << ":" << b << ":" << c << std::endl;
}
Call it with the following snippet:
template<typename Function, typename Tuple, size_t ... I>
auto call(Function f, Tuple t, std::index_sequence<I ...>)
{
return f(std::get<I>(t) ...);
}
template<typename Function, typename Tuple>
auto call(Function f, Tuple t)
{
static constexpr auto size = std::tuple_size<Tuple>::value;
return call(f, t, std::make_index_sequence<size>{});
}
Example:
int main()
{
std::tuple<int, double, int*> t;
//or std::array<int, 3> t;
//or std::pair<int, double> t;
call(f, t);
}
DEMO

Answer 3

This is a complete compilable version of Johannes' solution to awoodland's question, in the hope it may be useful to somebody. This was tested with a snapshot of g++ 4.7 on Debian squeeze.
###################
johannes.cc
###################
#include <tuple>
#include <iostream>
using std::cout;
using std::endl;
template<int ...> struct seq {};
template<int N, int ...S> struct gens : gens<N-1, N-1, S...> {};
template<int ...S> struct gens<0, S...>{ typedef seq<S...> type; };
double foo(int x, float y, double z)
{
return x + y + z;
}
template <typename ...Args>
struct save_it_for_later
{
std::tuple<Args...> params;
double (*func)(Args...);
double delayed_dispatch()
{
return callFunc(typename gens<sizeof...(Args)>::type());
}
template<int ...S>
double callFunc(seq<S...>)
{
return func(std::get<S>(params) ...);
}
};
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
#pragma GCC diagnostic ignored "-Wunused-variable"
#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
int main(void)
{
gens<10> g;
gens<10>::type s;
std::tuple<int, float, double> t = std::make_tuple(1, 1.2, 5);
save_it_for_later<int,float, double> saved = {t, foo};
cout << saved.delayed_dispatch() << endl;
}
#pragma GCC diagnostic pop
One can use the following SConstruct file
#####################
SConstruct
#####################
#!/usr/bin/python
env = Environment(CXX="g++-4.7", CXXFLAGS="-Wall -Werror -g -O3 -std=c++11")
env.Program(target="johannes", source=["johannes.cc"])
On my machine, this gives
g++-4.7 -o johannes.o -c -Wall -Werror -g -O3 -std=c++11 johannes.cc
g++-4.7 -o johannes johannes.o

Answer 4

Here is a C++14 solution.
template <typename ...Args>
struct save_it_for_later
{
std::tuple<Args...> params;
void (*func)(Args...);
template<std::size_t ...I>
void call_func(std::index_sequence<I...>)
{ func(std::get<I>(params)...); }
void delayed_dispatch()
{ call_func(std::index_sequence_for<Args...>{}); }
};
This still needs one helper function (call_func). Since this is a common idiom, perhaps the standard should support it directly as std::call with possible implementation
// helper class
template<typename R, template<typename...> class Params, typename... Args, std::size_t... I>
R call_helper(std::function<R(Args...)> const&func, Params<Args...> const&params, std::index_sequence<I...>)
{ return func(std::get<I>(params)...); }
// "return func(params...)"
template<typename R, template<typename...> class Params, typename... Args>
R call(std::function<R(Args...)> const&func, Params<Args...> const&params)
{ return call_helper(func,params,std::index_sequence_for<Args...>{}); }
Then our delayed dispatch becomes
template <typename ...Args>
struct save_it_for_later
{
std::tuple<Args...> params;
std::function<void(Args...)> func;
void delayed_dispatch()
{ std::call(func,params); }
};

Answer 5

This is a bit complicated to achieve (even though it is possible). I advise you to use a library where this is already implemented, namely Boost.Fusion (the invoke function). As a bonus, Boost Fusion works with C++03 compilers as well.

Answer 6

c++14 solution. First, some utility boilerplate:
template<std::size_t...Is>
auto index_over(std::index_sequence<Is...>){
return [](auto&&f)->decltype(auto){
return decltype(f)(f)( std::integral_constant<std::size_t, Is>{}... );
};
}
template<std::size_t N>
auto index_upto(std::integral_constant<std::size_t, N> ={}){
return index_over( std::make_index_sequence<N>{} );
}
These let you call a lambda with a series of compile-time integers.
void delayed_dispatch() {
auto indexer = index_upto<sizeof...(Args)>();
indexer([&](auto...Is){
func(std::get<Is>(params)...);
});
}
and we are done.
index_upto and index_over let you work with parameter packs without having to generate a new external overloads.
Of course, in c++17 you just
void delayed_dispatch() {
std::apply( func, params );
}
Now, if we like that, in c++14 we can write:
namespace notstd {
template<class T>
constexpr auto tuple_size_v = std::tuple_size<T>::value;
template<class F, class Tuple>
decltype(auto) apply( F&& f, Tuple&& tup ) {
auto indexer = index_upto<
tuple_size_v<std::remove_reference_t<Tuple>>
>();
return indexer(
[&](auto...Is)->decltype(auto) {
return std::forward<F>(f)(
std::get<Is>(std::forward<Tuple>(tup))...
);
}
);
}
}
relatively easily and get the cleaner c++17 syntax ready to ship.
void delayed_dispatch() {
notstd::apply( func, params );
}
just replace notstd with std when your compiler upgrades and bob is your uncle.

Answer 7

Thinking about the problem some more based on the answer given I've found another way of solving the same problem:
template <int N, int M, typename D>
struct call_or_recurse;
template <typename ...Types>
struct dispatcher {
template <typename F, typename ...Args>
static void impl(F f, const std::tuple<Types...>& params, Args... args) {
call_or_recurse<sizeof...(Args), sizeof...(Types), dispatcher<Types...> >::call(f, params, args...);
}
};
template <int N, int M, typename D>
struct call_or_recurse {
// recurse again
template <typename F, typename T, typename ...Args>
static void call(F f, const T& t, Args... args) {
D::template impl(f, t, std::get<M-(N+1)>(t), args...);
}
};
template <int N, typename D>
struct call_or_recurse<N,N,D> {
// do the call
template <typename F, typename T, typename ...Args>
static void call(F f, const T&, Args... args) {
f(args...);
}
};
Which requires changing the implementation of delayed_dispatch() to:
void delayed_dispatch() {
dispatcher<Args...>::impl(func, params);
}
This works by recursively converting the std::tuple into a parameter pack in its own right. call_or_recurse is needed as a specialization to terminate the recursion with the real call, which just unpacks the completed parameter pack.
I'm not sure this is in anyway a "better" solution, but it's another way of thinking about and solving it.
As another alternative solution you can use enable_if, to form something arguably simpler than my previous solution:
#include <iostream>
#include <functional>
#include <tuple>
void f(int a, double b, void* c) {
std::cout << a << ":" << b << ":" << c << std::endl;
}
template <typename ...Args>
struct save_it_for_later {
std::tuple<Args...> params;
void (*func)(Args...);
template <typename ...Actual>
typename std::enable_if<sizeof...(Actual) != sizeof...(Args)>::type
delayed_dispatch(Actual&& ...a) {
delayed_dispatch(std::forward<Actual>(a)..., std::get<sizeof...(Actual)>(params));
}
void delayed_dispatch(Args ...args) {
func(args...);
}
};
int main() {
int a=666;
double b = -1.234;
void *c = NULL;
save_it_for_later<int,double,void*> saved = {
std::tuple<int,double,void*>(a,b,c), f};
saved.delayed_dispatch();
}
The first overload just takes one more argument from the tuple and puts it into a parameter pack. The second overload takes a matching parameter pack and then makes the real call, with the first overload being disabled in the one and only case where the second would be viable.

Answer 8

My variation of the solution from Johannes using the C++14 std::index_sequence (and function return type as template parameter RetT):
template <typename RetT, typename ...Args>
struct save_it_for_later
{
RetT (*func)(Args...);
std::tuple<Args...> params;
save_it_for_later(RetT (*f)(Args...), std::tuple<Args...> par) : func { f }, params { par } {}
RetT delayed_dispatch()
{
return callFunc(std::index_sequence_for<Args...>{});
}
template<std::size_t... Is>
RetT callFunc(std::index_sequence<Is...>)
{
return func(std::get<Is>(params) ...);
}
};
double foo(int x, float y, double z)
{
return x + y + z;
}
int testTuple(void)
{
std::tuple<int, float, double> t = std::make_tuple(1, 1.2, 5);
save_it_for_later<double, int, float, double> saved (&foo, t);
cout << saved.delayed_dispatch() << endl;
return 0;
}

Answer 9

a lot of answers have been provided but I found them too complicated and not very natural. I did it another way, without using sizeof or counters.
I used my own simple structure (ParameterPack) for parameters to access the tail of parameters instead of a tuple. Then, I appended all the parameters from my structure into function parameters, and finnally, when no more parameters were to be unpacked, I run the function.
Here is the code in C++11, I agree that there is more code than in others answers, but I found it more understandable.
template <class ...Args>
struct PackParameters;
template <>
struct PackParameters <>
{
PackParameters() = default;
};
template <class T, class ...Args>
struct PackParameters <T, Args...>
{
PackParameters ( T firstElem, Args... args ) : value ( firstElem ),
rest ( args... ) {}
T value;
PackParameters<Args...> rest;
};
template <class ...Args>
struct RunFunction;
template <class T, class ...Args>
struct RunFunction<T, Args...>
{
template <class Function>
static void Run ( Function f, const PackParameters<T, Args...>& args );
template <class Function, class... AccumulatedArgs>
static void RunChild (
Function f,
const PackParameters<T, Args...>& remainingParams,
AccumulatedArgs... args
);
};
template <class T, class ...Args>
template <class Function>
void RunFunction<T, Args...>::Run (
Function f,
const PackParameters<T, Args...>& remainingParams
)
{
RunFunction<Args...>::template RunChild ( f, remainingParams.rest,
remainingParams.value );
}
template <class T, class ...Args>
template<class Function, class ...AccumulatedArgs>
void RunFunction<T, Args...>::RunChild ( Function f,
const PackParameters<T, Args...>& remainingParams,
AccumulatedArgs... args )
{
RunFunction<Args...>:: template RunChild ( f, remainingParams.rest,
args..., remainingParams.value );
}
template <>
struct RunFunction<>
{
template <class Function, class... AccumulatedArgs>
static void RunChild ( Function f, PackParameters<>, AccumulatedArgs... args )
{
f ( args... );
}
template <class Function>
static void Run ( Function f, PackParameters<> )
{
f ();
}
};
struct Toto
{
std::string k = "I am toto";
};
void f ( int i, Toto t, float b, std::string introMessage )
{
float res = i * b;
std::cerr << introMessage << " " << res << std::endl;
std::cerr << "Toto " << t.k << std::endl;
}
int main(){
Toto t;
PackParameters<int, Toto, float, std::string> pack ( 3, t, 4.0, " 3 * 4 =" );
RunFunction<int, Toto, float, std::string>::Run ( f, pack );
return 0;
}