Example on C++ data member aligment

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\p开发者_开发百科rojects\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?

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

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.

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

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)