[英]Is there an extensible way to initialize a struct from an array in C++?
struct SomeStruct {
int a;
int b;
int c;
int d;
};
void main() {
std::array<int, 4> arr {1, 2, 3, 4};
SomeStruct someStruct { ??? }; // <------ Initialize with arr? (Ideal would be {arr})
}
As seen in the code example, is there some shortcut to initialize someStruct with arr?如代码示例中所示,是否有一些快捷方式可以使用 arr 初始化 someStruct? Instead of resorting to:
而不是诉诸于:
SomeStruct someStruct {arr[0], arr[1], arr[2], arr[3]};
You can use std::apply
to create a parameter pack and use that in the initializer:您可以使用
std::apply
创建一个参数包并在初始化程序中使用它:
auto someStruct = std::apply([](auto&&... args){return SomeStruct{decltype(args)(args)...};}, arr);
You can put this in a function to use it comfortably:你可以把它放在一个函数中以舒适地使用它:
template<typename T, typename A>
constexpr T make_from_tuple_list_init(A&& a) {
return std::apply([](auto&&... args){return T{decltype(args)(args)...};}, std::forward<A>(a));
}
Then use it as然后将其用作
auto someStruct = make_from_tuple_list_init<SomeStruct>(arr);
This requires C++17 and #include<tuple>
, but can be implemented in earlier C++ versions (with some more effort) as well.这需要 C++17 和
#include<tuple>
,但也可以在早期的 C++ 版本中实现(需要更多的努力)。 Also in C++17 this is guaranteed to use copy elision, so there will be only one SomeStruct
constructed.同样在 C++17 中,这保证使用复制省略,因此只会构造一个
SomeStruct
。 Before C++17 some move constructions might be involved (although compilers were already permitted to and did implement return value optimization with the same effect).在 C++17 之前,可能会涉及一些移动构造(尽管编译器已经被允许并且确实实现了具有相同效果的返回值优化)。
std::apply
also has the benefit that it works with every type that implements std::get
and std::tuple_size
, meaning that it will work for example with std::array
, std::tuple
, std::pair
and other user-defined types with that interface. std::apply
也有一个好处,它适用于实现std::get
和std::tuple_size
所有类型,这意味着它将适用于例如std::array
、 std::tuple
、 std::pair
和其他具有该接口的用户定义类型。
One might want to make two versions of this function, one that uses T{...}
as above and one that uses T(...)
, so that one is able to choose between list-initialization and direct-initialization.一个人可能想要制作这个函数的两个版本,一个使用上面的
T{...}
,一个使用T(...)
,以便可以在列表初始化和直接初始化之间进行选择。
However, the T(...)
version has been implemented in the standard library since C++17 as std::make_from_tuple
.但是,自 C++17 以来,标准库中已将
T(...)
版本作为std::make_from_tuple
。 In C++20, aggregate initialization will also work with parentheses, so std::make_from_tuple
will work for your case then directly.在 C++20 中,聚合初始化也适用于括号,因此
std::make_from_tuple
将直接适用于您的情况。
For a C++14 implementation, take this example from cppreference's page on std::make_from_tuple
:对于 C++14 实现,请从
std::make_from_tuple
上的 cppreference 页面获取此示例:
namespace detail { template <class T, class Tuple, std::size_t... I> constexpr T make_from_tuple_impl( Tuple&& t, std::index_sequence<I...> ) { return T(std::get<I>(std::forward<Tuple>(t))...); } } // namespace detail template <class T, class Tuple> constexpr T make_from_tuple( Tuple&& t ) { return detail::make_from_tuple_impl<T>(std::forward<Tuple>(t), std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{}); }
and replace std::tuple_size_v<...>
with std::tuple_size<...>::value
.并将
std::tuple_size_v<...>
替换为std::tuple_size<...>::value
。
This is for the T(...)
version.这是针对
T(...)
版本的。 You would need to replace T(...)
with T{...}
to get the T{...}
version I implemented above.您需要将
T(...)
替换为T{...}
才能获得我上面实现的T{...}
版本。
You can define a constructor taking a pair of iterators.您可以定义一个带有一对迭代器的构造函数。 Sometimes simplicity wins over flexibility.
有时,简单胜过灵活性。
#include <array>
struct SomeStruct {
int a;
int b;
int c;
int d;
template<class InputIt>
SomeStruct(InputIt begin, InputIt end)
{
if (begin != end) a = *begin++;
if (begin != end) b = *begin++;
if (begin != end) c = *begin++;
if (begin != end) d = *begin++;
}
};
Demo: https://coliru.stacked-crooked.com/a/7e235a8022b048ac演示: https : //coliru.stacked-crooked.com/a/7e235a8022b048ac
You can use one of the Memory Transfer functions.您可以使用其中一种内存传输功能。 {memcpy, CopyMemory}
{memcpy, CopyMemory}
this is an example:这是一个例子:
#include <iostream>
#include <array>
#include <string.h>
using namespace std;
int main()
{
array<int,4> ar = {1,2,3,4};
struct myStruct
{
int a;
int b;
int c;
int d;
};
myStruct stc;
memcpy(&stc, ar.data(), sizeof(stc));
cout << "My struct elements are: " << stc.a << " - " << stc.b << " - "<< stc.c << " - " << stc.d << endl;
return 0;
}
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