structure alignment compiled ok in c,failed in c++

Question

I have the following souce about structure which compiled ok in gcc 4.4.6 :

struct st1 { char name[12]; int heartbeat ; double price ; int iFlag ; } ; struct st2 { struct st1 ; char pad[64 - sizeof(struct st1)] ; } __attribute__((aligned(64))) ; int main (void) { printf("length of struct st2=(%d)\n",sizeof(struct st2) ) ; } gcc -fms-extensions test1.c -o test1.exe ./test1.exe ===> length of struct st2=(64)

I copy test1.c to test1.cpp and try to compile as :

g++ -fms-extensions test1.cpp -o test1.exe

and then I got :

test1.cpp:23: error: invalid application of sizeof to incomplete type st2::st1

I know this error showes char pad[64 - sizeof(struct st1)] ; does not work in g++ , although it works in gcc , if I like this works in g++ , what can I do ?

Not sure why it got downvoted, seems like a perfectly reasonable question... — user1520427
– user1520427, Commented Apr 2, 2013 at 4:45

Nbr44 · Accepted Answer · 2013-04-02 04:48:06Z

In your struct st2:

struct st1 ;

This is a forward declaration of a struct st1. Here, you are basically saying to your compiler : "Hey, there's a struct st1 in the namespace of the struct st2 (hence the st2::st1), but I'm not going to give you its properties !" Since you are not providing its properties, the compiler will raise an "incomplete type" error : it can't know about the size of this type, and therefore cannot resolve your sizeof.

If you want to have an instance of your struct st1 in your struct st2, you should write:

struct st1 my_variable_name;

This will effectively add an instance of your struct st1 in your struct st2.

If you DON'T want an instance of your struct st1 in your struct st2, just remove this line - your compiler already knows about the struct st1, since it's declared right above.

Parker Kemp · Accepted Answer · 2013-04-02 04:43:48Z

Remove the "struct" and just use sizeof(st1). By adding "struct", you're telling the compiler to define a new struct st1 in the scope of st2. Hence why it says "st2::st1".

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