"unpacking" a tuple to call a matching function pointer
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?
Solution 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) ...);
}
// ...
Solution 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
Solution 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
Solution 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¶ms, 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¶ms)
{ 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); }
};
Solution 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.