[英]Compile time size of data member std::array in non-class template (C++14)
I need to statically assert that the compile-time size of a non-constexpr data member of type std::array
, say arr_
, of a non-template class is equal to a given (externally provided) constant.我需要静态断言非模板类的
std::array
类型的非 constexpr 数据成员的编译时大小,比如arr_
,等于给定的(外部提供的)常量。 The static assertion will be done from inside the class, meaning arr_
is accessible, but I cannot rely on any stored constant (nor a non-type template parameter) for its size.静态断言将从类内部完成,这意味着
arr_
是可访问的,但我不能依赖任何存储的常量(也不是非类型模板参数)的大小。 Ie, the assertion needs to rely solely on "some inspection" of the arr_
data member.即,断言需要仅依赖于
arr_
数据成员的“一些检查”。
I would basically be done if the constexpr
std::array<>::size()
/ std::array<>::max_size()
was static member functions ( decltype(arr_)::size()
/ decltype(arr_)::max_size()
) instead of a non-static member functions.如果
constexpr
std::array<>::size()
/ std::array<>::max_size()
是静态成员函数( decltype(arr_)::size()
/ decltype(arr_)::max_size()
我基本上会完成decltype(arr_)::max_size()
) 而不是非静态成员函数。
I have a working approach using function template argument deduction on a pointer-to-data-member for the arr_
member, but I'm wondering if there is an easier/neater approach.我有一种在
arr_
成员的指向数据成员的指针上使用函数模板参数推导的工作方法,但我想知道是否有更简单/更整洁的方法。
#include <array>
#include <cstddef>
// Defined/provided from elsewhere.
constexpr std::size_t kArraySize = 12U;
constexpr std::size_t kAnotherArraySize = 12U;
template <typename T, typename U, std::size_t N>
constexpr std::size_t size_of_data_member_array(std::array<T, N> U::*) {
return N;
}
class Foo {
std::array<int, kArraySize> arr_;
static_assert(size_of_data_member_array(&Foo::arr_) == kAnotherArraySize, "");
};
int main() {}
The standard provides a static version of array::size
under the name tuple_size
:该标准在名称
tuple_size
下提供了array::size
的静态版本:
#include <array>
#include <tuple> // for std::tuple_size_v
static_assert(std::tuple_size<decltype(arr_)>::value == kAnotherArraySize, "");
static_assert(std::tuple_size_v<decltype(arr_)> == kAnotherArraySize); // C++17
You can create an instance of an array with the same type of Foo::arr_
within the static assertion:您可以在静态断言中创建一个具有相同类型
Foo::arr_
的数组实例:
class Foo {
std::array<int, kArraySize> arr_;
static_assert(decltype(arr_){}.size() == kAnotherArraySize, "");
};
See this example .请参阅此示例。
Note : this works only if the array value type is a POD or has a default constexpr constructor.注意:这仅在数组值类型是 POD 或具有默认 constexpr 构造函数时才有效。
Just to offer another option, you can do partial specialization on template variables.只是为了提供另一种选择,您可以对模板变量进行部分专业化。
#include <array>
#include <cstddef>
// Defined/provided from elsewhere.
constexpr std::size_t kArraySize = 12U;
constexpr std::size_t kAnotherArraySize = 12U;
template <typename T>
constexpr std::size_t array_size = 0;
template <typename T, std::size_t N>
constexpr std::size_t array_size<std::array<T, N>> = N;
class Foo {
std::array<int, kArraySize> arr_;
static_assert(array_size<decltype(arr_)> == kAnotherArraySize, "");
};
int main() {}
To offer yet another option using SFINAE:使用 SFINAE 提供另一种选择:
#include <type_traits>
#include <array>
template <typename T>
constexpr bool is_array_v = false;
template <typename T, size_t N>
constexpr bool is_array_v<std::array<T, N>> = true;
template <typename Array, std::enable_if_t<is_array_v<Array>, int> = 0>
constexpr size_t array_size_v = std::tuple_size<Array>::value;
static_assert(array_size_v<std::array<int, 5>> == 5); // OK
static_assert(array_size_v<int> == 5); // Won't compile
Unlike the other proposals, this will catch misuse of array_size_v
at compile time, so entering an int
and other types which are not std::array
won't work.与其他提议不同,这将在编译时发现对
array_size_v
误用,因此输入int
和其他不是std::array
类型将不起作用。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.