Is this bad C++?
Author
I'm wondering about how C++ classes are stored on the heap and whether it can differ among compilers and platforms.
Let's say I have this simple class:
class A {
public:
int iar[3];
A() {}
};
A *pA = new A[2];
Now, is it safe to assume that ((int*)pA)[3] actually points to pA[1].ia[0]? And if so, is it an accepted scheme to use?
Next example:
class A {
public:
int x;
int y;
A() {}
};
A *pA = new A[2];
Same question; can you assume ((int*)pA)[2] points to pA[1].x?
That's compiler and architecture specific, because of padding and alignment.
But why would you want to do something like that?
[Edited by - noVum on September 27, 2004 7:16:20 PM]
But why would you want to do something like that?
[Edited by - noVum on September 27, 2004 7:16:20 PM]
Both your assumptions are correct. As to whether it being an acceptable scheme or not is a matter of taste i think. It's pretty safe to assume. But the c++ standard does not guarantee that soprt of behaviour, and those assumptions wont hold when you have derived classes.
Im sure Fruny has the c++ standanrd official technical lowdown on this though.
[edit]
also what noVum said...
[/edit]
Im sure Fruny has the c++ standanrd official technical lowdown on this though.
[edit]
also what noVum said...
[/edit]
Quote:Original post by androw
Now, is it safe to assume that ((int*)pA)[3] actually points to pA[1].ia[0]? And if so, is it an accepted scheme to use?
You can write &(pA[num / 3].iar[num % 3]) to get the address of the desired element.
BTW if you have a class without virtual functions and just public members, it behaves like a usual old struct. In this case your addressing scheme should work.
Making some mebers private should work either, but do not use virtual functions. Also pay attention to padding.
I use these a lot in Vertex Arrays and VBOs:
class Vertex{
public:
float x,y,z;
};
Vertex *mesh=Vertex[triangle_count*3];
glVertexPointerf(*,*,*,mesh);
Is completely correct with many compilers but i can see memory fragmentation cause problems with implementations where blocks can be allocated fragmentated (i have have never seen one, i don't even think there are any).
So i think (i don't know it completely sure) you can assume that, yes.
Btw, if not that would mean that this could get compiled wrong:
char txt[]="Blabla";
txt[1]='i'; // compiled as *(txt+(sizeof(char)*1))='i';
With memory fragmentation, txt[1] might not be the base address + the memory needed to store one char type value so it would probably crash.
class Vertex{
public:
float x,y,z;
};
Vertex *mesh=Vertex[triangle_count*3];
glVertexPointerf(*,*,*,mesh);
Is completely correct with many compilers but i can see memory fragmentation cause problems with implementations where blocks can be allocated fragmentated (i have have never seen one, i don't even think there are any).
So i think (i don't know it completely sure) you can assume that, yes.
Btw, if not that would mean that this could get compiled wrong:
char txt[]="Blabla";
txt[1]='i'; // compiled as *(txt+(sizeof(char)*1))='i';
With memory fragmentation, txt[1] might not be the base address + the memory needed to store one char type value so it would probably crash.
Author
Well, my intention with the desribed scheme are as follows:
I have a class called Vertex who's sole data member is an array of three floats describing the x, y and z components of a point in R3. Vertex also has some utility member functions for manipulating these components.
Now, in my program I need to be able to access an array of vertexes as an continuos array of floats i.e. {x1, y1, z1, x2, y2...}.
I've been thinking of several ways to accomplish this. But thought the described scheme was simple yet ugly. Does anyone know a preferred way of handling this?
I have a class called Vertex who's sole data member is an array of three floats describing the x, y and z components of a point in R3. Vertex also has some utility member functions for manipulating these components.
Now, in my program I need to be able to access an array of vertexes as an continuos array of floats i.e. {x1, y1, z1, x2, y2...}.
I've been thinking of several ways to accomplish this. But thought the described scheme was simple yet ugly. Does anyone know a preferred way of handling this?
The cleanest way would be copying the vertex data into a seperate buffer and passing that to he API, I suppose.
[edit]
You can also change you vector class to a struct. If you have no virtual methods and allocate the vectors as a block, your assumptions will hold true.
[/edit]
[edit]
You can also change you vector class to a struct. If you have no virtual methods and allocate the vectors as a block, your assumptions will hold true.
[/edit]
A struct is the same as a class, but with public access by default whereas a class has private access by default.
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