HP C/aC++ for PA-RISC Software manual Basic Example, Template Example

// Offset 0, 3 bytes padding // Offset 4, no padding

// Offset 16, 3 bytes padding // sizeof(S3)==20, alignment 4

// Offset 0, no padding // Offset 1, no padding // Offset 7, no padding

// sizeof(S4)==8, alignment 1

// Same as S1

// Offset 0, 3 bytes padding // Offset 4, no padding

// Offset 8, 3 bytes padding // sizeof(S5)==12, alignment 4

Basic Example

The following example illustrates the pack pragma and shows that it has no effect on class fields unless the class itself was defined under the pragma:

struct	S1 {
char		c1;	// Offset 0, 3 bytes padding
int	i;		// Offset 4, no padding
char		c2;	// Offset 8, 3 bytes padding
};			// sizeof(S1)==12, alignment 4
#pragma		pack 1
struct	S2 {
char		c1;	// Offset 0, no padding
int	i;		// Offset 1, no padding
char		c2;	// Offset 5, no padding
};			// sizeof(S2)==6, alignment 1

//S3 and S4 show that the pragma does not affect class fields

//unless the class itself was defined under the pragma. struct S3 {

char c1; S1 s; char c2;

};

struct S4 { char c1; S2 s; char c2;

};

#pragma pack

struct S5 { char c1; int i; char c2;

};

Template Example

If the pragma is applied to a class template, every instantiation of that class is influenced by the pragma value in effect when the template was defined. For example:

#pragma pack 1

template<class T> struct ST1 {

char c1;

T x;
char	c2;
};
#pragma	pack
ST1<int> obj;		// Same layout as S2 in the prior example
template <>		// Explicit specialization
struct ST1<void> {
char	c1;
char	c2;
};		// Undefined (unsupported) behavior
		// ST1 was defined under a #pragma pack 1 directive

100 Pragma Directives and Attributes