I have a struct called Point
typedef struct {
GLfloat x;
GLfloat y;
} Point;
create an array of Points:
Point *sPoints;
for(int i=0 ... // define sPoints
somewhere else, I want to alter variables on those points. Why does this work:
sPoints[100].x+=10;
but this doesn't:
Point pt = sPoints[100];
pt.x +=10;
is there any way to create a temporary variable that refers to the Point structure and allows me to set properties of that struct? The really strange thing is that in my non working code (pt.x +=10) I can actually read pt.x fine, I just can't seem to assign it... any help appreciated.
It doesn't work because in C, that operation:
Point pt = sPoints[100];
creates a copy of the item on the right hand side, whereas the former does not copy.
Related
I have two NSArrays, one of CLLocation and one of doubles (encased in objects) that I need to write to C vectors to draw a gradient MKPolyline as defined here (https://github.com/wdanxna/GradientPolyline). I tried to copy some of the code I saw there in preparation to call one of its functions:
points = malloc(sizeof(CLLocationCoordinate2D)*self.run.locations.array.count);
velocity = malloc(sizeof(float)*self.run.locations.array.count);
for(int i = 0; i<self.run.locations.array.count; i++){
points[i] = self.run.locations.array[i];
velocity[i] = [velocities[i] floatValue];
}
Here self.run.locations.array is an array of CLLocations.
Right now I can't even build the project because I have not declared the variables. But where/how do I declare these variables? I don't know C, and the project I am trying to use doesn't seem to include these declarations in a place I can find them.
points is an array of CLLocationCoordinate2D (dynamically allocated) so it should be a pointer to CLLocationCoordinate2D i.e.
CLLocationCoordinate2D *points;
velocity is an array of float so it should be declared as
float *velocity;
Alternatively you can do this
float velocity[];
if you prefer array syntax.
I'm creating a particle system renderer, the problem is that all my particle positions are encapsulated into classes that integrate them over time and do some other stuff. Instead of copying the values into a separate array to render each frame, I created a point struct like this that I can use to point to all the values once:
typedef struct
{
float *x, *y, *z;
} point;
I then malloc an array of these the same size as my array of particles.
Declaration at the top:
point *points;
Malloc:
points = malloc(sizeof(point) * [particles count]);
I then loop through all the particles and copy the addresses of each position to the corresponding pointer so:
for (int i = 0; i < [particles count]; ++i)
{
points[i].x = &[[particles objectAtIndex:i] getPosition].x;
points[i].y = &[[particles objectAtIndex:i] getPosition].y;
points[i].z = &[[particles objectAtIndex:i] getPosition].z;
}
My get position function just returns a vector struct (float x, y, z).
The first problem I encountered with this is that when I display the values like so:
NSLog(#"%f", *points[0].x);
Instead of reading of the value (say: 0.5), it displays it rounded and negated (so: -1.0).
I then get the data into my VBO like this:
glBufferData(GL_ARRAY_BUFFER, sizeof(point) * [particles count], &points[0], GL_DYNAMIC_DRAW);
I think this is also wrong but can't find any information on how to pass pointers into VBOs.
What you're doing is wrong because in C everything is passed by value, so when you dereference the .x member of the returned struct, you're doing it on a copy of the position struct, which will be popped from the stack if not assigned to any value.
You're design is wrong, you don't need a struct like this one:
typedef struct
{
float *x, *y, *z;
} point;
Rather one like this:
typedef struct
{
float x,y,z;
}
If you still want to assign it to it's original value, in a way that if the original struct's values change, also this struct is changed, then you need a getter who will return a pointer to the struct, and hold a pointer to it:
points[i] = [[particles objectAtIndex:i] getPositionPointer];
This way you're sure that your struct contains the same values of the original struct, but you're violating encapsulation. Which makes me think that maybe you don't really need to hold a pointer to original struct, but if there's a reason to do then I've shown you the way.
I have a struct Position, like this:
typedef struct Position { int x, y; } Position;
How may I pass it in NSObject performSelector:withObject:afterDelay:? Like this:
Position pos;
pos.x = pos.y = 1;
[self performSelector:#selector(foo:)
withObject:pos // ERROR
afterDelay:5.0f];
EDIT: changed code to fix typo
Uhm.. use a CGPoint and something like
[self performSelector:#selector(foo:)
withObject:[NSValue valueWithCGPoint:CGPointMake(pos.x, pos.y)]
afterDelay:5.0f];
And read it again as
NSValue v;
CGPoint point = [v CGPointValue];
or leave the Position class away completely, CGPoint does the same
You could wrap your custom type using NSValue class. The error is that you didn't provide an object reference to the method.
Try using NSValue's +(NSValue *)valueWithBytes:(const void *)value objCType:(const char *)type; class method. On the other side you can retrieve the value using -(void)getValue:(void *)buffer;.
preformSelector:withObject: accepts only objects as parameters, hence you'll have to implement your foo: method to accept an object. There are two ways to pass the struct as an object:
create a struct-like object or
wrap it into NSValue and unwrap it in the method.
Full answer, based on user756245's (which doesn't tell you how to use it, not a great deal of help). Also, Apple suggests you use a slightly different method these days, IIRC:
typedef myStruct { int a; } myStruct;
myStruct aLocalPointer = ... assign to it
[self performSelector:#selector(myOtherMethod:) withObject:[NSValue value:&aLocalPointer withObjCType:#encode(myStruct)] afterDelay:0.25];
This is most likely asking for trouble, but you can pass CGPoint as id by bridging it in this way:
withObject:(__bridge id)((void *)(&point))
This will lead to a crash if point goes out of scope and your selector tries to read it, though.
I have three questions surrounding what I think is the topic of structs in obj-c
1) Why is it that I often (or always) cannot take a member var that is a CGPoint or a CGRect and set the values one by one? I find I have to do:
CGPoint point;
point.x = someValue;
point.y = someOtherValue;
obj.myPoint = point;
instead of simply obj.myPoint.x = someValue etc.
2) Is this behavior that is consistent across all structs in obj-c?
3) Is there an easy way to add two CGPoints? It seems like there should already be, but I couldn't find one. I thought it'd be cumbersome if I'd have to use a temporary CGPoint to accumulate values between two CGPoints before setting the dest var to the temp var (because of not being able to just do pointA.x += pointB.x (same for y).
1) From #sb in an answer to Cocoa Objective-c Property C structure assign fails
That won't accomplish anything, because [t member] returns a struct, which is an "r-value", ie. a value that's only valid for use on the right-hand side of an assignment. It has a value, but it's meaningless to try to change that value.
Basically you just have to live with the fact that you can't set the fields of struct directly when returned from a function.
2) Yes
3) Unfortunately I don't think there is a built-in convenience method for adding two CGPoint. If you find your self doing this frequently you can make your own:
CGPoint CGPointAdd(CGPoint p1, CGPoint p2)
{
return CGPointMake(p1.x + p2.x, p1.y + p2.y);
}
and then use it like:
obj.pointA = CGPointAdd(obj.pointA, pointB);
not as elegant as obj.pointA.X += ... but sometimes life isn't fair.
This is perfectly normal. The 'obj' owns that var and has getters and setters, you can not modify parts of that variable.
The best thing to do is copy the struct and modify whatever you need.
Also note you can use the CGPointMake(x, y) function (and the same for all CG structs), which is much easier.
To update this is easiest:
CGPoint point = obj.myPoint;
point.x += 10.0f;
obj.myPoint = point;
Obj-C 2.0 hides the getters and setters which looks like this:
CGPoint point = [obj myPoint];
point.x += 10.0f;
[obj setMyPoint:point];
I've got object_getInstanceVariable to work as here however it seems to only work for floats, bools and ints not doubles. I do suspect I'm doing something wrong but I've been going in circles with this.
float myFloatValue;
float someFloat = 2.123f;
object_getInstanceVariable(self, "someFloat", (void*)&myFloatValue);
works, and myFloatValue = 2.123
but when I try
double myDoubleValue;
double someDouble = 2.123f;
object_getInstanceVariable(self, "someDouble", (void*)&myDoubleValue);
I get myDoubleValue = 0. If I try to set myDoubleValue before the function eg. double myDoubleValue = 1.2f, the value is unchanged when I read it after the object_getInstanceVariable call. Setting myIntValue to some other value before the getinstancevar function above returns 2 as it should, ie. it has been changed.
then I tried
Ivar tmpIvar = object_getInstanceVariable(self, "someDouble", (void*)&myDoubleValue);
If I do ivar_getName(tmpIvar) I get "someDouble", but myDoubuleValue = 0 still! Then I try ivar_getTypeEncoding(tmpIvar) and I get "d" as it should be.
So to summarize, if typeEncoding = float, it works, if it is a double, the result is not set but it correctly reads the variable and the return value (Ivar) is also correct.
I must be doing something basic wrong that I cant see so I'd appreciate if someone could point it out.
object_getInstanceVariable is a confused little function. It is documented that the last parameter is a void ** parameter—that is, you pass the address of a void * variable and get a pointer to the instance variable—but it is implemented as if it was a void * parameter—that is, you pass the address of the variable that you want to hold a copy of the instance variable. The problem is that the implementation ignores the size of the instance variable and just does a pointer copy. So anything that's the same size as a pointer will work perfectly. If you're running on a 32-bit architecture, only the high 32 bits will be copied. (You should witness the same behavior with a long long instance variable as well.)
The solution is to use the primary API, key-value coding, using -valueForKey:.
The other solution: If you wanted to write a fixed version, say as a category for NSObject, it would look something like this:
#implementation NSObject (InstanceVariableForKey)
- (void *)instanceVariableForKey:(NSString *)aKey {
if (aKey) {
Ivar ivar = object_getInstanceVariable(self, [aKey UTF8String], NULL);
if (ivar) {
return (void *)((char *)self + ivar_getOffset(ivar));
}
}
return NULL;
}
#end
Then your code would look like this:
double myDoubleValue = *(double *)[self instanceVariableForKey:#"someDouble"];
What about using valueForKey:?
NSNumber * value = [self valueForKey:[NSString stringWithUTF8String:ivar_getName(tmpIvar)]];
NSLog(#"Double value: %f", [value doubleValue];
Note: this requires you to have a "someFloat" method. If you want to use setValue:forKey:, you'll also need the "setSomeFloat:" method. This is easily implemented by declaring the ivar as an #property and synthesizing it.