C++ access member of a struct inside a union

Question

I'm trying to write a C++ program for an Atmel Microcontroller.

I need to use some of the functions that were already written in the ASF library, and I'm doing this by copying some of the code into C++ functions.

I get this error on compilation:

'union Pm' has no member named 'PM_CPUMASK'

The Pm type union looks something like this:

typedef union {
 struct {
    ....  
 } bf;
 struct {
  ....
  RwReg   PM_CPUMASK;
  ....
 } reg;
} Pm;`

In the ASF code, the member PM_CPUMASK is accessed like this:

unsigned int mask = *(&PM->PM_CPUMASK + busId);

But I get an error. I think this is valid in C, but in C++ I would need to access the named struct and then the actual member. Using *(&PM->reg.PM_CPUMASK...) I get no error, but is there a way to activate this style of member accessing in C++ compiler?

BTW, there's no naming conflicts in the members of the 2 structs. Thanks.

To me it doesn't look like a valid C either. You could try to remove inner struct names (`bf` and `reg`). It would make the code work if your your compiler supports anonymous structs as a (non-standard) extension. (Otherwise the only other option is to `#define PM_CPUMASK reg.PM_CPUMASK`, but it's an ugly hack.) — HolyBlackCat, Jun 05 '17 at 14:51

score 3 · Accepted Answer · answered Jun 05 '17 at 15:02

3

I would guess that you copy-pasted the code and then modified it. The original code was something like this

typedef union {
 struct {
    ....  
 };
 struct {
  ....
  RwReg   PM_CPUMASK;
  ....
 };
} Pm;

What is anonymous struct, that is a compiler extension, and there is no default extensions like this for C++ compiler, see here.

And then you added names to those structs. That is why you have to use another identifier to access the member.

Sorry if my guessing is wrong. But the point is that you have to use an additional name there.

answered Jun 05 '17 at 15:02

Yuki

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You are somewhat right. I copied the code from a C function, but the union is defined in an already included file, which i will not modify. But when i checked the same file in a C project, Pm was no longer defined as a union of structs, but just as a struct. So it appears that i have to modify a bit the code to work with the C++ files, even if they're very similar. – user3808318 Jun 05 '17 at 15:09
@user3808318 always rember that C and C++ are two separate languages, even if they may look similar they sometimes have different semantics for identical source. – Jesper Juhl Jun 05 '17 at 15:25
@JesperJuhl I actually do not know any case, where for the same source compiled with C and C++ there will be different semantics, that would be interesting to see, could point out to some examples? Because I know that C is just a subset of C++. With that said, when you port C code to C++ there might be some syntax features absent in favor to new ones, but a case where compiling C code with C++ compiler will have different behavior (i.e. semantics) would be nice to see. – Yuki Jun 05 '17 at 16:15
@Yuki read this for a start: https://en.m.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B - especially the "Constructs that behave differently in C and C++" section. – Jesper Juhl Jun 05 '17 at 17:21

C++ access member of a struct inside a union

1 Answers1