why i can't assign I[5] ?

[xixi]

#include <iostream.h>
class A{
private:
int x,y ;
public:
A(int a=0,int b=0){y=b;x=a;}
void init(int a,int b){x=a;y=b;}
void show(){cout<<x<<" "<<y<<endl;}
};
void main()
{
A l[5]={(1,1),(2,2),(3,3),(4,4),(5,5)};
A *p;
p=l;
l[3].init(8,3);
p->init(3,4);
for(int i=0;i<5;i++)
(*(p+i)).show();
}

when i execute this code,I[0] was shown (1.0),but I assign it (1,1),i[0] was not assigned.so was i[1],i[2]...

What can i do to assign i[5]?thanks very much.

[kuphryn]

Index count begins with 0. That is a fundamental concept of computer architecture.

If you want to start with array[1], array[2], etc. then you should ignore array[0]. In your example, create six arrays.

Kuphryn

[xixi]

In my program ,i assign array like this:

A l[5]={(1,1),(2,2),(3,3),(4,4),(5,5)};

but it execute like this :

1 0
2 0
3 0
4 0
5 0

it like:
y of class A is not assigned.but x of class A is assigned.

why?thank you .What can i do to assign y of class A?

[kuphryn]

One solution is a loop.

for (unsigned int i = 0; i < 5; ++i)
I[i].Create(i + i, i + i);

Kuphryn

[Graham]

Try this:

A l[5]={A(1,1), A(2,2), A(3,3), A(4,4), A(5,5)};

[dumah]

Although it ugly, there is a way to use Kuphryn's loop and Graham's constructor method in one.

This code relies on operator new[] that's provided in the <new> header and then using placement new to initialise each object's construction in a loop. Unfortunately it looks nasty, it's not easy to see what's going on, and you nust control the destruction and memory freeing yourself

#include <new>
#include <iostream>


class A{

	int x,y ;
public:
	A(int a=0,int b=0){y=b;x=a;}
	void init(int a,int b){x=a;y=b;}
	void show(){std::cout << x << " " << y << std::endl;}
};

int main(){

	const int nArrSz = 5;
	
	A *l = static_cast<A*>(operator new[] (nArrSz * sizeof(A)));//Allocate raw memory
	if(!l){
		std::cout << "Error" << std::endl;
		return 1;
	}	
	
	for(int i = 0;i < nArrSz;i++)
		new (l+i) A(i+1,i+1);//Call constructor for each object in array
	
	
	A *p;
	p=l;
	l[3].init(8,3);
	p->init(3,4);
	
	for(int i=0;i<5;i++)
		(*(p+i)).show();
		
	for(int i = 0;i < nArrSz;i++)
		l[i].~A();//Call destuctor for each object in array
		
	operator delete[] (l);//free memory	
		
}

Enjoy!

[AnthonyMai]

[Y: Solution exploiting struct-like behaviour]

What the OP needs to do is:
1.Change the "private" to public. If you need to initialize the class member from an array, it needs to be accessible so you can't hide with "private".
2.Remove any constructor you declared. The compiler will decide how to initialize the object, not you.
3.Change the round bracket in your parameter list to curly bracket.

This is how the working code looks like:

//...
class A
{
public:
	int x;
	int y;
public:
	void init(int a,int b){x=a;y=b;}
	void show() {printf("%d %d\r\n", x, y);}
};

int main(int argc, char* argv[])
{
	A l[5] = {{1,1},{2,2},{3,3},{4,4},{5,5}};
//...
}

[Yves : cleaned up the post a bit]

[PaulWendt]

Originally posted by AnthonyMai
I just don't understand why the OP posted something so simple and straightforward and no one can give a correct and straight answer?

I didn't find a problem with some of the answers here; maybe
they didn't want to totally alter his class design. What was wrong
with Graham's response?

What the OP needs to do is:
1.Change the "private" to public. If you need to initialize the class member from an array, it needs to be accessible so you can't hide with "private".

... and what if the OP wants to use the information-hiding ideas
inherent in C++? It can be difficult to make an abstract data
type of others can get access to your private members.

2.Remove any constructor you declared. The compiler will decide how to initialize the object, not you.

... and what if the OP wants to provide some other form of
initialization in this class? I don't think he should be forced to
use inheritance to create such a simplistic class.

Just as an aside, just because you can change a bunch of things
in order to "implement a solution", it doesn't mean it's an ideal
solution. Frankly, I wouldn't want to compromise my class design
so radically like you suggest; I'd rather use any number of
alternative methods.

--Paul

[dumah]

Originally posted by AnthonyMai
I just don't understand why the OP posted something so simple and straightforward and no one can give a correct and straight answer?

As I see it, there is no correct & straight answer. Your idea may indeed be closest to how the OP was attempting this, but it involves him changing how the class operates and the privilege levels of the data members, so therefore its not the definitive answer.

Only the OP can decide which of the methods presented fits closest to what he/ she wants to do

[dumah]

Apologies to previous poster. Didnt see your response when I began mine.

[Graham]

Originally posted by AnthonyMai
I just don't understand why the OP posted something so simple and straightforward and no one can give a correct and straight answer?

Point out the error in my answer, please.

What the OP needs to do is:
1.Change the "private" to public. If you need to initialize the class member from an array, it needs to be accessible so you can't hide with "private".

So he has to violate encapsulation and expose the members of his class to random modification?

2.Remove any constructor you declared. The compiler will decide how to initialize the object, not you.

Tosh.

3.Change the round bracket in your parameter list to curly bracket.

This is how the working code looks like:

//...
class A
{
public:
	int x;
	int y;
public:
	void init(int a,int b){x=a;y=b;}
	void show() {printf("%d %d\r\n", x, y);}
};

int main(int argc, char* argv[])
{
	A l[5] = {{1,1},{2,2},{3,3},{4,4},{5,5}};
//...
}

So why didn't you also change the "class" to "struct" and remove the access specifiers and member functions, since yours is a "C" answer, not a "C++" answer?

[PaulWendt]

Originally posted by dumah
Apologies to previous poster. Didnt see your response when I began mine.

Oh don't worry about it If you take a couple of minutes to type
our your reply, I could have hit "Submit" in that time. The more
correct responses the merrier.

--Paul

[dumah]

Originally posted by PaulWendt


Oh don't worry about it If you take a couple of minutes to type
our your reply, I could have hit "Submit" in that time. The more
correct responses the merrier.

--Paul

LOL - Yeah, looking at it I was really slow typing that out - work can be such a distracting place when you are trying to post on forums

[AnthonyMai]

Originally posted by AnthonyMai
I just don't understand why the OP posted something so simple and straightforward and no one can give a correct AND straight answer?

Graham said:
Point out the error in my answer, please.

Graham, you did not give an answer that is correct AND straight. By "straight" I mean it should be concise and elegant. You solution calls the constuctor individually. Obviously if there are 1000 elements in the array you would call constructor and destructor 100 times respectively, if there are 10000 you will call 10000 times. So it is not straight at all.

So why didn't you also change the "class" to "struct" and remove the access specifiers and member functions, since yours is a "C" answer, not a "C++" answer?

With due respect I do NOT think you can write a struct with member function in C. Structs with member functions is a feature available in C++ but not C.

[AnthonyMai]

... and what if the OP wants to provide some other form of
initialization in this class? I don't think he should be forced to
use inheritance to create such a simplistic class.

"Some other form of initialization" is construct first and then initialize.

It is indeed a very simplistic class, the very fact that it can be initialized using some POD data array tells you it is a very C like simplistic class.

Just as an aside, just because you can change a bunch of things
in order to "implement a solution", it doesn't mean it's an ideal
solution. Frankly, I wouldn't want to compromise my class design
so radically like you suggest; I'd rather use any number of
alternative methods.

Since it is a simple class, please quit the talk of class design, and proper data encapsulation, and a bunch of other high level OOP concepts. These concepts may be good when the design is complicated. But they are not important here when things are rather simple and can be done in the simplest way.

[Yves : small edits]