can not understand Sizeof result with a struct var

MRTgang (2)
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typedef struct Element Element;
struct Element
{
    char x;
    double* y;
};


int main()
{
    Element a;

    printf("%d\n",sizeof(Element ));
    return 0;
}

this one with y pointer gives 8

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typedef struct Element Element;
struct Element
{
    char x;
    double y;
};


int main()
{
    Element a;

    printf("%d\n",sizeof(Element ));
    return 0;
}


ths one with a normal y variable gives 12
Last edited on
Disch (13742)
Your pointers are probably 4 bytes (try doing sizeof(double*))
Your doubles are probably 8 bytes (try doing sizeof(double))

The single char is 1 byte, but the struct will get padded up to the nearest 4-byte boundary for alignment reasons.
Catfish3 (666)
Member data in structures and classes is often word-aligned, for performance reasons.
This means there's padding (unused space) between your member data x and y.

A common way to disable padding is:
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#pragma pack(1)
struct Element
{
    char x;
    double y;
}
#pragma pack()

Peter87 (11256)
Catfish3 wrote:
A common way to disable padding

Note that this relies on compiler extensions and is not standard C++.
Catfish3 (666)
Note that this relies on compiler extensions and is not standard C++.

You're welcome to show a standard C++ method, if there is one.
MRTgang (2)
thank you guys .. reading about " padding" .made me figuring it out
Disch (13742)
Note that this relies on compiler extensions and is not standard C++.


Also note that this is likely to deteriorate performance. Things are padded for a reason.
Cubbi (4774)
could also simply crash ( on certain non-Intel platforms )
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#include <iostream>

#pragma pack(1)
struct Element
{
    char x;
    double y;
};
#pragma pack()

int main()
{
    Element e = {'a', 3.14};
    std::cout << "Size of Element is " << sizeof e << '\n'
              << "Enter a double: ";
    std::cin >> e.y;
    std::cerr << e.y << '\n';
}
 
$ ./test
Size of Element is 9
Enter a double: 1.23
Bus Error
Last edited on
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