含联合体成员的结构体是否为POD？特定结构体实例问询

Great questions! Let's dive into the rules around C++ POD types to answer these clearly.

Short answer: It depends—but yes, it’s entirely possible, as long as both the struct itself and its contained union meet the strict criteria for being a POD type.

In C++11 and later, a POD (Plain Old Data) type must satisfy two core requirements:

• Trivial type: The type has compiler-generated (not user-defined) trivial default, copy, and move constructors; trivial copy and move assignment operators; and a trivial destructor. Trivial types can be safely copied with memcpy and have no virtual functions or virtual base classes.
• Standard-layout type: The type has no virtual functions/virtual base classes, no non-static reference members, all non-static data members share the same access control (e.g., all public), and all non-static data members and base classes are themselves standard-layout types.

For a struct with a union member to be POD:

• The struct must check all the trivial/standard-layout boxes (no virtuals, consistent access control, trivial special members, etc.)
• The union inside it must also be trivial and standard-layout (i.e., all union members are trivial/standard-layout, no virtuals/references, etc.)

First, let's recap your type definitions for clarity:

struct Struct1 { void* myPtr; int myInt; float mySpecialFloat; };
struct Struct2 { void* myPtr; int myInt; int mySpecialInt; };
struct MyWrapStruct { 
    bool myBool; 
    union { 
        struct { void* myPtr; int myInt; }; 
        Struct1 myStruct1; 
        Struct2 myStruct2; 
    } myStructs; 
};

We’ll analyze each type step by step (note: we’re assuming your compiler supports anonymous structs, a common extension used by GCC, Clang, and MSVC—strict standard C++ doesn’t allow anonymous structs, but most compilers permit them):

Struct1 and Struct2 #

Both are definitely POD:

• They only contain fundamental types (void*, int, float/int), which are all POD.
• No virtual functions, virtual base classes, or reference members.
• All members use default public access control (consistent across the struct).
• All special member functions (constructor, destructor, assignment) are compiler-generated and trivial.
• They satisfy both trivial and standard-layout requirements.

The anonymous union myStructs #

This union is also trivial and standard-layout:

• All its members (the anonymous struct, Struct1, Struct2) are POD (so trivial + standard-layout).
• No virtual functions, virtual base classes, or reference members.
• The union’s special member functions are compiler-generated and trivial (since all its members are trivial).

MyWrapStruct #

Putting it all together, MyWrapStruct is POD:

• It has two members: a bool (POD) and the union we confirmed is trivial/standard-layout.
• No virtuals, references, or mixed access control (all members are public by default).
• All special member functions are compiler-generated and trivial.
• It meets both trivial and standard-layout criteria fully.

内容的提问来源于stack exchange，提问作者wip