keywords: C++, Byte Alignment, pack pragma

Example

Testing code:

class C1
{
    char a;
    char b;
    int c;
};

class C2
{
    char a;
    int c;
    char b;
};

class C3
{
    char a;
    char b;
    int c;

    void fun1() {}
};

class C4
{
    char a;
    char b;
    int c;

    virtual void fun1() {}
};

class C5
{
    char a;
    char b;
    int c;

    virtual void fun1() = 0;
};

int main(int argc, char* args[])
{
    printf("C1 size:%d\n", sizeof(C1));
    printf("C2 size:%d\n", sizeof(C2));
    printf("C3 size:%d\n", sizeof(C3));
    printf("C4 size:%d\n", sizeof(C4));
    printf("C5 size:%d\n", sizeof(C5));
}

Output(x64):

C1 size:8
C2 size:12
C3 size:8
C4 size:16
C5 size:16

Output(x86):

C1 size:8
C2 size:12
C3 size:8
C4 size:12
C5 size:12
What’s the default value in bytes used of packing.

(Optional) Specifies the value, in bytes, to be used for packing. If the compiler option /Zp isn't set for the module, the default value for n is 8. Valid values are 1, 2, 4, 8, and 16. The alignment of a member is on a boundary that’s either a multiple of n, or a multiple of the size of the member, whichever is smaller.

Origin: pack pragma
https://docs.microsoft.com/en-us/cpp/preprocessor/pack?view=vs-2019


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