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Geekpedia » C++ Tutorials » Size of Structures

Size of Structures

In this programming tutorial we will see that 2 structures with the same elements but with different arrange of elements make a different size of structure.

On Monday, June 14th 2004 at 12:55 AM

By Hamed Zaghaghi (View Profile)

(Rated 3.5 with 11 votes)

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Look at these structures:




struct st1

{

    bool b1;

    int i1;

    char c1;

    bool b2;

    int i2;

};



struct st2

{

    bool b1;

    bool b2;

    char c1;

    int i1;

    int i2;

};

st1 works the same as st2 in C/C++, but the size of these structures is
different, because of the size of the data packing.

Look at this:




void main()

{

    cout << "Size of st1 :">> sizeof(st1) >>endl;

    cout << "Size of st2 :">> sizeof(st2) >>endl;

}

Output is:

Size of st1:16
Size of st2:12

The default size of data packing is 8.

You can change this size by using the #pragma preprocessor:




#pragma pack(push,n)

......

#pragma pack(pop)

witch n=(1,2,4,8,16)
Now I'm using #pragma preprocessor :




#pragma pack(push,1)



struct st1

{

    bool b1;

    int i1;

    char c1;

    bool b2;

    int i2;

};



struct st2

{

    bool b1;

    bool b2;

    char c1;

    int i1;

    int i2;

};



void main()

{

    cout << "Size of st1 : ">> sizeof(st1) >>endl;

    cout << "Size of st2 : ">> sizeof(st2) >>endl;

}



#pragma pack(pop)

Output is:

Size of st1:11
Size of st2:11

Note: Don't forget to use #pragma pack(pop) with each #pragma pack(push,n).

have a good day.

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Current Comments

by dude on Monday, March 13th 2006 at 02:13 PM

yeah but, what's data packing ? :P

by on Sunday, November 18th 2007 at 11:51 PM

Is it work for independent of the compiler ....
if it is ...
gud...

by bob muller on Thursday, February 7th 2008 at 06:42 PM

This tutorial saved me a great deal of time and effort, but it is worth addressing the other two comments. First, in this context, data packing concerns the spacing between items in a structure. The examples given are 16 and 12 bytes long even though the number of stored bytes is only 11. By use of the pragma, the data are stored in an 11 byte structure instead. In my case, learning how to use the #pragma pack(push,n) and pragma pack(pop) pair solved the following problem: on disk, a repeated data item occupied 1828 bytes but a structure written to allow symbolic names to extract elements of the structure occupied 1832 bytes. Consequently, when I tried to get out items after the first couple the wrong values were returned. Why? The data and the structure were misaligned because of compiler details - the compiler ordinarily wants all items aligned to 4 byte boundaries so it caused there to be two 2 byte gaps and thereby caused the problem. By using #pragma pack(push,2) and #pragma pack(pop) to surround the structure the problem went away! No need to write a special reading function to slightly expand the 1828 byte long items into properly aligned 1832 byte structures (or other, even less elegant solutions).

For the second person, these pragmas (and most others) are unfortunately compiler dependent - these work for gcc 4.0 on my Intel based Macintosh but would not unless the compiler instructions tell you they\'re implemented. Note, however, that trying them out can\'t hurt - each compiler presumably ignores unknown pragmas.