question about struct

[abu]

I think it very easy but it makes me confused

struct Fileheader
{ 
unsigned short Type; // BM Identifier
unsigned long  Size; // total length in bytes
unsigned short Reserved1; // image id or revision 
unsigned short Reserved2;
unsigned long  OffBits;// offset to start of actual pixel data
Fileheader()
{
Reserved1=0;
Reserved2=0;
}
};
int main()
{
	cout<<sizeof(Fileheader)<<endl;
}

The Output is 16 <---2+4+2+2+4=??
I can't believe so I check again

struct Fileheader
{ 
unsigned short Type; // BM Identifier
unsigned short  Size; // total length in bytes
unsigned short Reserved1; // image id or revision 
unsigned short Reserved2;
unsigned long  OffBits;// offset to start of actual pixel data
Fileheader()
{
Reserved1=0;
Reserved2=0;
}
};
int main()
{
	cout<<sizeof(Fileheader)<<endl;
}

The output is 12=2+2+2+2+4...accurate but


So it means that we can't have a struct having size of 14??

[Prelude]

>So it means that we can't have a struct having size of 14??
It depends on how your implementation aligns memory as well as how structures are padded to meet those alignment requirements.

[abu]

You mean that b/c of I'm out of memory??I don't think so
I want to load bitmap file into my program so the struct must have the length enough.
so anyone can explain me

[Prelude]

>You mean that b/c of I'm out of memory??
No, I don't. I mean that structures are padded for alignment reasons. This changes the size of the structure instances. That is why you don't get the size that you expect and cannot get the size that you want. If your code requires that the size of a structure instance be 14 bytes, you must read the file manually and assign your input to the correct member.

[Salem]

Depending on your compiler, you could try

#pragma pack(1)
struct Fileheader
{ 
  unsigned short Type; // BM Identifier
  unsigned short  Size; // total length in bytes
  unsigned short Reserved1; // image id or revision 
  unsigned short Reserved2;
  unsigned long  OffBits;// offset to start of actual pixel data
};
#pragma pack()