Programming Forums > Application Development > C++

aloksave · Sep 17th, 2005, 2:13 PM

From what i know,In C the sizeof a structure or the obj of that structure is same as the same of the size of sum of all the data members of the structure..
I have worked it on AIX Unix using a Visual Age compiler and noticed that it works that way,i think similar holds good for C++ class as well unless the class is empty(empty class always shows sizeof as 1,while a program with empty structure doent even pass compilation....wonder why both these results??)
however i get different results if i compile it on Microsoft Visual C++,the size obtained is much more...here are the codes are tried out...

(Toggle Plain Text)

class foo
{
private:
	int a;
	char b;
	int c;
}obj1;


void main()
{
	cout<<"sizeof int ["<<sizeof(int)<<"]"<<endl;
	cout<<"sizeof char ["<<sizeof(char)<<"]"<<endl;
	cout<<"expected sizeof class ["<<sizeof(int)+sizeof(int)+sizeof(char)<<"]"<<endl;
	cout<<"sizeof class ["<<sizeof(foo)<<"]"<<endl;
	cout<<"sizeof obj ["<<sizeof(obj1)<<"]"<<endl;
}

OUTPUT:
sizeof int [4]
sizeof char [1]
expected sizeof class [9]
sizeof class [12]
sizeof obj [12]

code for c:

(Toggle Plain Text)

struct foo
{
	int a;
	char b;
	int c;
}obj1;


void main()
{
	printf("sizeof int [%d]\n",sizeof(int));
	printf("sizeof int [%d]\n",sizeof(char));
    printf("expected sizeof structure[%d]\n",sizeof(int)+sizeof(int)+sizeof(char)); 
	printf("sizeof structure [%d]\n",sizeof(struct foo));
	printf("sizeof int [%d]\n",sizeof(obj1));
}

output:
sizeof int [4]
sizeof int [1]
expected sizeof structure[9]
sizeof structure [12]
sizeof int [12]

can anyone explain why this occurs,this is got to do with some padding concept,is it? what is this padding concept?also please try to anser about the empty class and struct...
P.S: too many questions in this post,may be silly too but im learning in to it and with ur replies hopefully i will be able to learn faster...

DaWei · Sep 17th, 2005, 3:58 PM

It has everything to do with padding and alignment. Often, you may specify to what degree that takes place.

Quote:

Originally Posted by MSDN

When the sizeof operator is applied to a class, struct, or union type, the result is the number of bytes in an object of that class, struct, or union type, plus any padding added to align members on word boundaries. (The /Zp [pack structure members] compiler option and the pack pragma affect alignment boundaries for members.) The sizeof operator never yields 0, even for an empty class.

Sep 17th, 2005, 2:13 PM	#1
aloksave Programmer Join Date: Sep 2005 Posts: 33 Rep Power: 0	sizeof struct and class From what i know,In C the sizeof a structure or the obj of that structure is same as the same of the size of sum of all the data members of the structure.. I have worked it on AIX Unix using a Visual Age compiler and noticed that it works that way,i think similar holds good for C++ class as well unless the class is empty(empty class always shows sizeof as 1,while a program with empty structure doent even pass compilation....wonder why both these results??) however i get different results if i compile it on Microsoft Visual C++,the size obtained is much more...here are the codes are tried out... (Toggle Plain Text) class foo { private: int a; char b; int c; }obj1; void main() { cout<<"sizeof int ["<<sizeof(int)<<"]"<<endl; cout<<"sizeof char ["<<sizeof(char)<<"]"<<endl; cout<<"expected sizeof class ["<<sizeof(int)+sizeof(int)+sizeof(char)<<"]"<<endl; cout<<"sizeof class ["<<sizeof(foo)<<"]"<<endl; cout<<"sizeof obj ["<<sizeof(obj1)<<"]"<<endl; } class foo { private: int a; char b; int c; }obj1; void main() { cout<<"sizeof int ["<<sizeof(int)<<"]"<<endl; cout<<"sizeof char ["<<sizeof(char)<<"]"<<endl; cout<<"expected sizeof class ["<<sizeof(int)+sizeof(int)+sizeof(char)<<"]"<<endl; cout<<"sizeof class ["<<sizeof(foo)<<"]"<<endl; cout<<"sizeof obj ["<<sizeof(obj1)<<"]"<<endl; } OUTPUT: sizeof int [4] sizeof char [1] expected sizeof class [9] sizeof class [12] sizeof obj [12] code for c: (Toggle Plain Text) struct foo { int a; char b; int c; }obj1; void main() { printf("sizeof int [%d]\n",sizeof(int)); printf("sizeof int [%d]\n",sizeof(char)); printf("expected sizeof structure[%d]\n",sizeof(int)+sizeof(int)+sizeof(char)); printf("sizeof structure [%d]\n",sizeof(struct foo)); printf("sizeof int [%d]\n",sizeof(obj1)); } struct foo { int a; char b; int c; }obj1; void main() { printf("sizeof int [%d]\n",sizeof(int)); printf("sizeof int [%d]\n",sizeof(char)); printf("expected sizeof structure[%d]\n",sizeof(int)+sizeof(int)+sizeof(char)); printf("sizeof structure [%d]\n",sizeof(struct foo)); printf("sizeof int [%d]\n",sizeof(obj1)); } output: sizeof int [4] sizeof int [1] expected sizeof structure[9] sizeof structure [12] sizeof int [12] can anyone explain why this occurs,this is got to do with some padding concept,is it? what is this padding concept?also please try to anser about the empty class and struct... P.S: too many questions in this post,may be silly too but im learning in to it and with ur replies hopefully i will be able to learn faster...

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Programming Forums > Application Development > C++

sizeof struct and class