C ++ efficient programming NOTE 2: byte alignment in struct

# Include <iostream. h>

struct A {
char A ; long B ; char C ; long d ;
};
struct B {
char A ; char C ; long B ; long d ;
};

# Pragma pack (push, 1)
StructC{
CharA;LongB;CharC;LongD;
};
# Pragma pack (POP)

Struct D {
Char * A ; Char * B ;
};
// Use the structure of the bit domain
Struct Bitfield {
Unsigned A1 : 11 ; // Long 1
Unsigned A2 : 11 ;
Unsigned B1 : 10 ;
Unsigned A3 : 11 ; // Long 2
Unsigned A4 : 11 ;
Unsigned B2 : 10 ;
};

Void Main ( Void )
{
Cout < "Size of :" < Sizeof ( A ) < "Bytes" < Endl ;
Cout < "Size of B :" < Sizeof ( B ) < "Bytes" < Endl ;
Cout < "Size of C :" < Sizeof ( C ) < "Bytes" < Endl ;
Cout < "Size of D :" < Sizeof ( D ) < "Bytes" < Endl ;
Cout < "Size of bitfield :" < Sizeof ( Bitfield ) < "Bytes" < Endl ;
}

 

Running result:

Size of A: 16 bytes
Size of B: 12 bytes
Size of C: 10 bytes
Size of D: 8 bytes
Size of bitfield: 8 bytes
The reason why A, B, and C are different is byte alignment. # The Pragma pack (push, 1) command allows the compiler to temporarily adjust the alignment to 1 byte.
Note that char * generally occupies 4 bytes.