Ví dụ về tất cả các trường hợp không phải POD với các hiệu ứng static_asserttừ C ++ 11 đến C ++ 17 và POD
std::is_pod đã được thêm vào C ++ 11, vì vậy bây giờ chúng ta hãy xem xét tiêu chuẩn đó.
std::is_podsẽ bị xóa khỏi C ++ 20 như được đề cập tại https://stackoverflow.com/a/48435532/895245 , hãy cập nhật thông tin này khi có hỗ trợ thay thế.
Các hạn chế POD đã trở nên ngày càng thoải mái hơn khi tiêu chuẩn phát triển, tôi hướng đến việc bao gồm tất cả các thư giãn trong ví dụ thông qua ifdefs.
libstdc ++ có rất ít thử nghiệm tại: https://github.com/gcc-mirror/gcc/blob/gcc-8_2_0-release/libstdc%2B%2B-v3/testsuite/20_util/is_pod/value.cc nhưng nó chỉ là quá ít Người bảo trì: vui lòng hợp nhất điều này nếu bạn đọc bài viết này. Tôi lười kiểm tra tất cả các dự án thử nghiệm C ++ được đề cập tại: /software/199708/is-there-a-compliance-test-for-c-compilers
#include <type_traits>
#include <array>
#include <vector>
int main() {
#if __cplusplus >= 201103L
// # Not POD
//
// Non-POD examples. Let's just walk all non-recursive non-POD branches of cppreference.
{
// Non-trivial implies non-POD.
// https://en.cppreference.com/w/cpp/named_req/TrivialType
{
// Has one or more default constructors, all of which are either
// trivial or deleted, and at least one of which is not deleted.
{
// Not trivial because we removed the default constructor
// by using our own custom non-default constructor.
{
struct C {
C(int) {}
};
static_assert(std::is_trivially_copyable<C>(), "");
static_assert(!std::is_trivial<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// No, this is not a default trivial constructor either:
// https://en.cppreference.com/w/cpp/language/default_constructor
//
// The constructor is not user-provided (i.e., is implicitly-defined or
// defaulted on its first declaration)
{
struct C {
C() {}
};
static_assert(std::is_trivially_copyable<C>(), "");
static_assert(!std::is_trivial<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
}
// Not trivial because not trivially copyable.
{
struct C {
C(C&) {}
};
static_assert(!std::is_trivially_copyable<C>(), "");
static_assert(!std::is_trivial<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
}
// Non-standard layout implies non-POD.
// https://en.cppreference.com/w/cpp/named_req/StandardLayoutType
{
// Non static members with different access control.
{
// i is public and j is private.
{
struct C {
public:
int i;
private:
int j;
};
static_assert(!std::is_standard_layout<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// These have the same access control.
{
struct C {
private:
int i;
int j;
};
static_assert(std::is_standard_layout<C>(), "");
static_assert(std::is_pod<C>(), "");
struct D {
public:
int i;
int j;
};
static_assert(std::is_standard_layout<D>(), "");
static_assert(std::is_pod<D>(), "");
}
}
// Virtual function.
{
struct C {
virtual void f() = 0;
};
static_assert(!std::is_standard_layout<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// Non-static member that is reference.
{
struct C {
int &i;
};
static_assert(!std::is_standard_layout<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// Neither:
//
// - has no base classes with non-static data members, or
// - has no non-static data members in the most derived class
// and at most one base class with non-static data members
{
// Non POD because has two base classes with non-static data members.
{
struct Base1 {
int i;
};
struct Base2 {
int j;
};
struct C : Base1, Base2 {};
static_assert(!std::is_standard_layout<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// POD: has just one base class with non-static member.
{
struct Base1 {
int i;
};
struct C : Base1 {};
static_assert(std::is_standard_layout<C>(), "");
static_assert(std::is_pod<C>(), "");
}
// Just one base class with non-static member: Base1, Base2 has none.
{
struct Base1 {
int i;
};
struct Base2 {};
struct C : Base1, Base2 {};
static_assert(std::is_standard_layout<C>(), "");
static_assert(std::is_pod<C>(), "");
}
}
// Base classes of the same type as the first non-static data member.
// TODO failing on GCC 8.1 -std=c++11, 14 and 17.
{
struct C {};
struct D : C {
C c;
};
//static_assert(!std::is_standard_layout<C>(), "");
//static_assert(!std::is_pod<C>(), "");
};
// C++14 standard layout new rules, yay!
{
// Has two (possibly indirect) base class subobjects of the same type.
// Here C has two base classes which are indirectly "Base".
//
// TODO failing on GCC 8.1 -std=c++11, 14 and 17.
// even though the example was copy pasted from cppreference.
{
struct Q {};
struct S : Q { };
struct T : Q { };
struct U : S, T { }; // not a standard-layout class: two base class subobjects of type Q
//static_assert(!std::is_standard_layout<U>(), "");
//static_assert(!std::is_pod<U>(), "");
}
// Has all non-static data members and bit-fields declared in the same class
// (either all in the derived or all in some base).
{
struct Base { int i; };
struct Middle : Base {};
struct C : Middle { int j; };
static_assert(!std::is_standard_layout<C>(), "");
static_assert(!std::is_pod<C>(), "");
}
// None of the base class subobjects has the same type as
// for non-union types, as the first non-static data member
//
// TODO: similar to the C++11 for which we could not make a proper example,
// but with recursivity added.
// TODO come up with an example that is POD in C++14 but not in C++11.
}
}
}
// # POD
//
// POD examples. Everything that does not fall neatly in the non-POD examples.
{
// Can't get more POD than this.
{
struct C {};
static_assert(std::is_pod<C>(), "");
static_assert(std::is_pod<int>(), "");
}
// Array of POD is POD.
{
struct C {};
static_assert(std::is_pod<C>(), "");
static_assert(std::is_pod<C[]>(), "");
}
// Private member: became POD in C++11
// /programming/4762788/can-a-class-with-all-private-members-be-a-pod-class/4762944#4762944
{
struct C {
private:
int i;
};
#if __cplusplus >= 201103L
static_assert(std::is_pod<C>(), "");
#else
static_assert(!std::is_pod<C>(), "");
#endif
}
// Most standard library containers are not POD because they are not trivial,
// which can be seen directly from their interface definition in the standard.
// /programming/27165436/pod-implications-for-a-struct-which-holds-an-standard-library-container
{
static_assert(!std::is_pod<std::vector<int>>(), "");
static_assert(!std::is_trivially_copyable<std::vector<int>>(), "");
// Some might be though:
// /programming/3674247/is-stdarrayt-s-guaranteed-to-be-pod-if-t-is-pod
static_assert(std::is_pod<std::array<int, 1>>(), "");
}
}
// # POD effects
//
// Now let's verify what effects does PODness have.
//
// Note that this is not easy to do automatically, since many of the
// failures are undefined behaviour.
//
// A good initial list can be found at:
// /programming/4178175/what-are-aggregates-and-pods-and-how-why-are-they-special/4178176#4178176
{
struct Pod {
uint32_t i;
uint64_t j;
};
static_assert(std::is_pod<Pod>(), "");
struct NotPod {
NotPod(uint32_t i, uint64_t j) : i(i), j(j) {}
uint32_t i;
uint64_t j;
};
static_assert(!std::is_pod<NotPod>(), "");
// __attribute__((packed)) only works for POD, and is ignored for non-POD, and emits a warning
// /programming/35152877/ignoring-packed-attribute-because-of-unpacked-non-pod-field/52986680#52986680
{
struct C {
int i;
};
struct D : C {
int j;
};
struct E {
D d;
} /*__attribute__((packed))*/;
static_assert(std::is_pod<C>(), "");
static_assert(!std::is_pod<D>(), "");
static_assert(!std::is_pod<E>(), "");
}
}
#endif
}
GitHub ngược dòng .
Đã thử nghiệm với:
for std in 11 14 17; do echo $std; g++-8 -Wall -Werror -Wextra -pedantic -std=c++$std pod.cpp; done
trên Ubuntu 18.04, GCC 8.2.0.