Enforcing structure order?

[Sebastiani]

Do compilers guarantee that the structure generated by your code will place the variables in the exact order you declared them? And if not, can you give an example for how to do this on, say, your compiler?

I am having this problem:

struct something {
int a;
int b;
int c;
int d;
};


struct something s;

memset(&s, 0, sizeof(s));
int value = 22;
s.a = value;

if(memcmp(&s, &value, sizeof(int)) == 0) {
  cout << "Indeed..." << endl;
 }
else {
  cout << "Afraid not..." << endl;
 }

The output is always negative.

However, I found the value at offset + 8, and thus this printed positive:

if(memcmp(&s + 8, &value, sizeof(int)) == 0) {
  cout << "Indeed..." << endl;
 }
else {
  cout << "Afraid not..." << endl;
 }

How can I enforce the order of placement?

[moi]

ahhh! cout in the C board!!!!

/me shuns the code to the C++ board


anyway, the test snippet prints "indeed" on my machine, which is what i expected it to print. so i can't really help you....

[Sebastiani]

Well, to be honest, that was not the whole story. In essence, I am using a template to hold any type of object, which can vary in size, of course. The template structure also has variable which stores the size of the object contained within, such that I might be able to query the template at runtime, by simply calling memcmp on the structure starting at the 0th offset, using the size variable as a ruler, so to speak. Naturally, the compliler wants to put known types first, since it will make generating the code for accessing that structure much simpler. What I want to do is overide this behaviour such to align the stored object on the 0th offset.

[moi]

on your platform, does this trimmed down code still exhibit the undesidered behavior like the original? (it doesnt on mine).

[moi]

Originally posted by Salem
between members of the structure. Though this is the first I've heard of a structure being padded before the first element.

the (very good) C book i have in front of me says that padding before first element is NOT allowed. so i would expect that this is one of those class thingies, with hidden class info.

[Sebastiani]

Thank you Salem for your help. It worked!