C++ Returning several values from a function Structured Bindings
Example
C++17
C++17 introduces structured bindings, which makes it even easier to deal with multiple return types, as you do not need to rely upon std::tie() or do any manual tuple unpacking:
std::map<std::string, int> m;
// insert an element into the map and check if insertion succeeded
auto [iterator, success] = m.insert({"Hello", 42});
if (success) {
// your code goes here
}
// iterate over all elements without having to use the cryptic 'first' and 'second' names
for (auto const& [key, value] : m) {
std::cout << "The value for " << key << " is " << value << '\n';
}
Structured bindings can be used by default with std::pair, std::tuple, and any type whose non-static data members are all either public direct members or members of an unambiguous base class:
struct A { int x; };
struct B : A { int y; };
B foo();
// with structured bindings
const auto [x, y] = foo();
// equivalent code without structured bindings
const auto result = foo();
auto& x = result.x;
auto& y = result.y;
If you make your type "tuple-like" it will also automatically work with your type. A tuple-like is a type with appropriate tuple_size, tuple_element and get written:
namespace my_ns {
struct my_type {
int x;
double d;
std::string s;
};
struct my_type_view {
my_type* ptr;
};
}
namespace std {
template<>
struct tuple_size<my_ns::my_type_view> : std::integral_constant<std::size_t, 3>
{};
template<> struct tuple_element<my_ns::my_type_view, 0>{ using type = int; };
template<> struct tuple_element<my_ns::my_type_view, 1>{ using type = double; };
template<> struct tuple_element<my_ns::my_type_view, 2>{ using type = std::string; };
}
namespace my_ns {
template<std::size_t I>
decltype(auto) get(my_type_view const& v) {
if constexpr (I == 0)
return v.ptr->x;
else if constexpr (I == 1)
return v.ptr->d;
else if constexpr (I == 2)
return v.ptr->s;
static_assert(I < 3, "Only 3 elements");
}
}
now this works:
my_ns::my_type t{1, 3.14, "hello world"};
my_ns::my_type_view foo() {
return {&t};
}
int main() {
auto[x, d, s] = foo();
std::cout << x << ',' << d << ',' << s << '\n';
}
