当仅在运行时知道索引时,从所需类型的索引默认构造std :: variant的最简单方法是什么?换句话说,我想写:
const auto indx = std::variant<types...>{someobject}.index();
//...somewhere later, indx having been passed around...
std::variant<types...> var = variant_from_index(indx);
///var is now set to contain a default constructed someobject
请注意,无法将indx
设为constexpr,因此std::in_place_index
在这里不起作用。
这里的问题当然是,由于尚不知道在编译时要从types...
调用哪个构造函数,因此基本上必须以某种方式构建所有可能的构造函数的表(或从中复制默认构造变量)编译时,然后在运行时访问。这里显然有一些模板魔术,但是最干净的方法是什么?
我尝试了以下操作[on coliru,但是索引序列似乎出了错(最后的打印给出了2 0 0
),我对原因感到困惑:
Edit:它的工作原理如下,我的constexpr数组初始化错误。所以现在的问题是,有没有更整洁的方法来做到这一点?
#include <variant>
#include <iostream>
using var_t = std::variant<int, float, const char *>;
//For debug
template<class ...types>
struct WhichType;
template<class T, class U>
struct default_variants;
template<class...Params, std::size_t... I>
struct default_variants<std::variant<Params...>, std::index_sequence<I...>> {
using variant_t = std::variant<Params...>;
//Uncomment to see the index sequence
//WhichType<std::index_sequence<I...>> idx{};
constexpr static variant_t variants[sizeof...(Params)]{variant_t{std::in_place_index<I>}...};
constexpr static std::size_t indices[sizeof...(Params)]{I...};
};
template<class T>
struct default_variants_builder;
template<class...Params>
struct default_variants_builder<std::variant<Params...>> {
using indices = std::make_index_sequence<sizeof...(Params)>;
using type = default_variants<std::variant<Params...>, indices>;
};
int main() {
using builder_t = typename default_variants_builder<var_t>::type;
var_t floatvar{1.2f};
var_t variant2 = builder_t::variants[floatvar.index()];
std::cout << "Contained " << floatvar.index() << "; Now contains " << variant2.index() << "\n";
}
不确定这是否非常优雅,但我认为它可行:
#include <variant>
#include <iostream>
template<typename V, std::size_t N = std::variant_size_v<V>>
struct variant_by_index {
V make_default(std::size_t i) {
if (i >= std::variant_size_v<V>) {
throw std::invalid_argument("bad type index.");
}
constexpr size_t index = std::variant_size_v<V> - N;
if (i == index) {
return std::variant_alternative_t<index, V>();
} else {
return variant_by_index<V, N - 1>().make_default(i);
}
}
};
template<typename V>
struct variant_by_index<V, 0> {
V make_default(std::size_t i) {
throw std::invalid_argument("bad type index.");
}
};
using var_t = std::variant<int, float, const char *>;
int main() {
variant_by_index<var_t> type_indexer;
var_t my_var_0 = type_indexer.make_default(0);
std::cout << "my_var_0 has int? " << std::holds_alternative<int>(my_var_0) << "\n";
var_t my_var_1 = type_indexer.make_default(1);
std::cout << "my_var_1 has float? " << std::holds_alternative<float>(my_var_1) << "\n";
try {
var_t my_var_1 = type_indexer.make_default(3);
} catch(const std::invalid_argument&) {
std::cout << "Could not create with type 3.\n";
}
return 0;
}