问题
Given the following code:
struct Window{
void show();
//stuff
}w1, w2, w3;
struct Widget{
void show();
//stuff
}w4, w5, w6;
struct Toolbar{
void show();
//stuff
}t1, t2, t3;
I want to show a bunch of items:
for (auto &obj : {w3, w4, w5, t1})
obj.show();
However this does not compile since the std::initializer_list<T> in the for-loop cannot deduce T and in fact there is not really a T that would fit. I don't want to create a type erasure type because of the amount of code required and the unnecessary runtime overhead. How do I correctly write my loop so that the type of obj is deduced for every item in the conceptual list separately?
回答1:
In C++17 or better you'd use fold expressions, to "walk through" your heterogenous arguments applying the member function:
auto Printer = [](auto&&... args) {
(args.show(), ...);
};
Printer(w1, w2, w3, w4, w5, w6, t1, t2, t3);
Demo
You can read more on this in my blog
回答2:
boost::fusion is awesome but oldskool - it caters for the deficiencies in c++03.
c++11's variadic template expansion to the rescue!
#include <iostream>
struct Window{
void show() {
std::cout << "Window\n";
}
//stuff
}w1, w2, w3;
struct Widget{
void show() {
std::cout << "Widget\n";
}
//stuff
}w4, w5, w6;
struct Toolbar{
void show()
{
std::cout << "Toolbar\n";
}
//stuff
}t1, t2, t3;
template<class...Objects>
void call_show(Objects&&...objects)
{
using expand = int[];
(void) expand { 0, ((void)objects.show(), 0)... };
}
auto main() -> int
{
call_show(w3, w4, w5, t1);
return 0;
}
expected output:
Window
Widget
Widget
Toolbar
another, more generic way (requires c++14):
// note that i have avoided a function names that look like
// one in the standard library.
template<class Functor, class...Objects>
void for_all(Functor&& f, Objects&&... objects)
{
using expand = int[];
(void) expand { 0, (f(std::forward<Objects>(objects)), 0)... };
}
called like so:
for_all([](auto& thing) { thing.show(); }, w3, w4, w5, t1);
回答3:
Another option is to use boost::tuple or std::tuple and boost::fusion::for_each algorithm:
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/adapted/boost_tuple.hpp>
boost::fusion::for_each(
boost::tie(w1, w2, w3, w4, w5, w6, t1, t2, t3), // by reference, not a copy
[](auto&& t) { t.show(); }
);
Just out of curiosity, compared the generated assembly output of Richard Hodges's method with the above. With gcc-4.9.2 -Wall -Wextra -std=gnu++14 -O3 -march=native the produced assembly code is identical.
回答4:
Based on https://stackoverflow.com/a/6894436/3484570 this works without creating an extra function, boost or inheritance.
Header:
#include <tuple>
#include <utility>
template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I == sizeof...(Tp), void>::type
for_each(const std::tuple<Tp...> &, FuncT) // Unused arguments are given no names.
{ }
template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I < sizeof...(Tp), void>::type
for_each(const std::tuple<Tp...>& t, FuncT f)
{
f(std::get<I>(t));
for_each<I + 1, FuncT, Tp...>(t, f);
}
template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I == sizeof...(Tp), void>::type
for_each(std::tuple<Tp...> &&, FuncT) // Unused arguments are given no names.
{ }
template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I < sizeof...(Tp), void>::type
for_each(std::tuple<Tp...>&& t, FuncT f)
{
f(std::get<I>(t));
for_each<I + 1, FuncT, Tp...>(std::move(t), f);
}
.cpp:
struct Window{
void show(){}
//stuff
}w1, w2, w3;
struct Widget{
void show(){}
//stuff
}w4, w5, w6;
struct Toolbar{
void show(){}
//stuff
}t1, t2, t3;
int main() {
for_each(std::tie(w3, w4, w5, t1), [](auto &obj){
obj.show();
});
}
回答5:
Window, Widget and Toolbar share common interface, so you can create abstract class and make other classes inherit from it:
struct Showable {
virtual void show() = 0; // abstract method
};
struct Window: Showable{
void show();
//stuff
}w1, w2, w3;
struct Widget: Showable{
void show();
//stuff
}w4, w5, w6;
struct Toolbar: Showable{
void show();
//stuff
}t1, t2, t3;
Then, you can create array of pointers to Showable, and iterate over it:
int main() {
Showable *items[] = {&w3, &w4, &w5, &t1};
for (auto &obj : items)
obj->show();
}
See it working online
回答6:
I recommend Boost.Hana, which IMHO is the best and most flexible template meta-programming library available.
#include <boost/hana/ext/std/tuple.hpp>
#include <boost/hana.hpp>
namespace hana = boost::hana;
hana::for_each(std::tie(w3, w4, w5, t1), [](auto& obj) { obj.show(); });
回答7:
I think boost::variant is worth mentioning. All the more it has chances to become std::variant in C++17.
int main()
{
std::vector<boost::variant<Window*, Widget*, Toolbar*>> items = { &w1, &w4, &t1 };
for (const auto& item : items)
{
boost::apply_visitor([](auto* v) { v->show(); }, item);
}
return 0;
}
回答8:
A late answer but here is general solution with C++14 which works like the boost::fusion::for_each but doesn't require Boost:
#include <tuple>
namespace detail {
template<typename Tuple, typename Function, std::size_t... Is>
void tuple_for_each_impl(Tuple&& tup, Function&& fn, std::index_sequence<Is...>) {
using dummy = int[];
static_cast<void>(dummy {
0, (static_cast<void>(fn(std::get<Is>(std::forward<Tuple>(tup)))), 0)...
});
}
}
template<typename Function, typename... Args>
void tuple_for_each(std::tuple<Args...>&& tup, Function&& fn) {
detail::tuple_for_each_impl(std::forward<std::tuple<Args...>>(tup),
std::forward<Function>(fn), std::index_sequence_for<Args...>{});
}
int main() {
tuple_for_each(std::tie(w1, w2, w3, w4, w5, w6, t1, t2, t3), [](auto&& arg) {
arg.show();
});
}
If you want to achieve more or less the same thing without the std::tuple, you can create a single-function variant of the above code:
#include <utility>
template<typename Function, typename... Args>
void va_for_each(Function&& fn, Args&&... args) {
using dummy = int[];
static_cast<void>(dummy {
0, (static_cast<void>(fn(std::forward<Args>(args))), 0)...
});
}
int main() {
auto action = [](auto&& arg) { arg.show(); };
va_for_each(action, w1, w2, w3, w4, w5, w6, t1, t2, t3);
}
The drawback of the second example is that it requires to specify the processing function first, therefore doesn't have the same look like the well known std::for_each. Anyway with my compiler (GCC 5.4.0) using -O2 optimization level, they produce the same assembly output.
来源：https://stackoverflow.com/questions/34314193/iterating-over-different-types