Type punning a struct in C and C++ via a union

Question

I\'ve compiled this in gcc and g++ with pedantic and I don\'t get a warning in either one:

#include 
#include 
#include 

        

Answer 1

In C:

struct a and struct b are not compatible types. Even in

typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;

s1 and s2 are not compatible types. (See example in 6.7.8/p5.) An easy way to identify non-compatible structs is that if two struct types are compatible, then something of one type can be assigned to something of the other type. If you would expect the compiler to complain when you try to do that, then they are not compatible types.

Therefore, struct a * and struct b * are also not compatible types, and so struct a and struct b do not share a common initial subsequence. Your union-punning is instead governed by the same rule for union punning in other cases (6.5.2.3 footnote 95):

If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation.

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In C++, struct a and struct b also do not share a common initial subsequence. [class.mem]/p18 (quoting N4140):

Two standard-layout structs share a common initial sequence if corresponding members have layout-compatible types and either neither member is a bit-field or both are bit-fields with the same width for a sequence of one or more initial members.

[basic.types]/p9:

If two types T1 and T2 are the same type, then T1 and T2 are layout-compatible types. [ Note: Layout-compatible enumerations are described in 7.2. Layout-compatible standard-layout structs and standard-layout unions are described in 9.2. —end note ]

struct a * and struct b * are neither structs nor unions nor enumerations; therefore they are only layout-compatible if they are the same type, which they are not.

It is true that ([basic.compound]/p3)

Pointers to cv-qualified and cv-unqualified versions (3.9.3) of layout-compatible types shall have the same value representation and alignment requirements (3.11).

But that does not mean those pointer types are layout-compatible types, as that term is defined in the standard.