Example on C++ data member aligment

Question

I have tried to implement Stack Overflow question C++ Data Member Alignment and Array Packing. Here is the code:

#include <stdio.h>
#include <stddef.h>
typedef struct {
    unsigned char a;
    unsigned char b;
    unsigned char c;
}foo;

typedef struct{
    unsigned short i;
    unsigned char a;
    unsigned char b;
    unsigned char c;
} Bar;

typedef struct {foo f[5];} f_b;
typedef struct {Bar[5];} b_f;
#define ALIGNMENT_OF(t) offsetof( struct { char x; t test; }, test )

int main(void){
    printf("Foo:: Size: %d; Alignment: %d\n", sizeof(foo), ALIGNMENT_OF(foo));
    printf("Bar:: Size: %d; Alignment: %d\n", sizeof(Bar), ALIGNMENT_OF(Bar));
    printf("F_B:: Size: %d; Alignment: %d\n", sizeof(f_b), ALIGNMENT_OF(f_B));
    printf("B_F:: Size: %d; Alignment: %d\n", sizeof(b_f), ALIGNMENT_OF(b_f));
    return 0;
}

But here are the mistakes:

1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(16): error C2059: syntax error : '['
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(16): error C2238: unexpected token(s) preceding ';'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2332: 'struct' : missing tag name
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing '(' before ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2143: syntax error : missing ';' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2227: left of '->test' must point to class/struct/union/generic type
1>          type is 'int'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(19): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2332: 'struct' : missing tag name
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing '(' before ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2143: syntax error : missing ';' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2227: left of '->test' must point to class/struct/union/generic type
1>          type is 'int'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(20): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2332: 'struct' : missing tag name
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing '(' before ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2143: syntax error : missing ';' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2065: 'f_B' : undeclared identifier
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2146: syntax error : missing ';' before identifier 'test'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2227: left of '->test' must point to class/struct/union/generic type
1>          type is 'int'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(21): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2332: 'struct' : missing tag name
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing '(' before ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ')' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2143: syntax error : missing ';' before '{'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2227: left of '->test' must point to class/struct/union/generic type
1>          type is 'int'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
1>c:\users\david\documents\visual studio 2010\projects\data_packing\data_packing.cpp(22): error C2059: syntax error : ')'
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

But in code it does not show me a red line which indicates an error. How do I fix this problem?

Do you really expect us to help you fix your glaringly obvious errors? Actually, your compiler does a pretty good job at pointing out the errors...what have you tried so far? — Greg S, Sep 02 '10 at 12:36
@Greg compiler yes does good job but sometimes we can't find way to solve this problems so ask expierenced people to help what is surprise? — , Sep 02 '10 at 12:47
It seems to me that a little more effort could have been put into solving a simple compile error before posting it here. If you're really stuck, looking up error codes on MSDN can sometimes be enlightening. — Patrick Johnmeyer, Sep 02 '10 at 12:49
ok i understand so please dont donwvote more it is question not war :) — , Sep 02 '10 at 12:52
-1 for pasting compiler errors that are obvious. Please clearly explain what you're having trouble with, 'here are mistakes' is not sufficient. — Sam Miller, Sep 02 '10 at 12:53
@c-p downvotes are not war, they're just people letting you know how they feel. — Philip Potter, Sep 02 '10 at 12:57
@c-programmer: put at least some effort into formatting your code, ask specific questions and almost everyone here will be more than glad to help you (several of your last questions are requests to fix errors pointed out by your compiler...). — Greg S, Sep 02 '10 at 13:03
@Greg S - It looks like it's davit-datuashvili back under another user name. You can see the same paths in this question for example http://stackoverflow.com/questions/3357500/syntax-error-with-template-in-c, and he evidently hasn't learned from the suspension of his other account as the questions haven't improved. Don't expect anything to change. — Greg Beech, Sep 03 '10 at 08:30

score 5 · Answer 1 · answered Sep 02 '10 at 12:31

5

See where it says \data_packing.cpp(16):? That 16 in parentheses is a line number.

answered Sep 02 '10 at 12:31

Philip Potter

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score 3 · Answer 2 · answered Sep 02 '10 at 12:34

3

In the struct b_f, you created an array of Bar without a name. Also, in C++, you don't need all that typedef stuff.

answered Sep 02 '10 at 12:34

Puppy

144,682
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1

Yeah, wtf is up with all the `typedef struct`'s? This isn't C, and the example linked to didn't do it that way. – T.E.D. Sep 02 '10 at 13:02

score 2 · Accepted Answer · edited Oct 30 '11 at 17:03

2

For starters

Bar[5]

is not valid code. It should be something like

Bar f[5]

Furthermore, that macro doesn't make sense. You might try this instead:

template< class T >
struct testStruct{ char x; T test; };

#define ALIGNMENT_OF(t) offsetof( testStruct< t >, test )

And finally there's a typo:

ALIGNMENT_OF(f_B) //should be f_b

edited Oct 30 '11 at 17:03

Peter Mortensen

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answered Sep 02 '10 at 12:32

stijn

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inline function would still be better; macros in C++ are :(( – tenfour Sep 02 '10 at 12:46
even better would probably be to look how boost does the alignment_of struct implementation ad use that, it's really nice. – stijn Sep 02 '10 at 12:49

score 0 · Answer 4 · answered Sep 02 '10 at 13:09

The pragma pack and pop might be helpful. The example below will align each member on one byte boundaries. In VS this can also be set on a project level by changing the project setting C++/CodeGeneration/StructMemberAlignment.

#pragma pack(push, 1)
struct test {
    int a;
    char b[3];
    double c;
};
#pragma pack(pop)

Example on C++ data member aligment

4 Answers4