can not understand Sizeof result with a struct var

[MRTgang]

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

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

[Disch]

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]

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:

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

[Peter87]

Catfish3 wrote:

A common way to disable padding

Note that this relies on compiler extensions and is not standard C++.

[Catfish3]

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]

thank you guys .. reading about " padding" .made me figuring it out

[Disch]

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]

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