Cairo memory leak on draw - c++

Question

I've written a drawCircle function below but as more circles are drawn/redrawn, memory usage increases greatly, so I'm assuming theres a memory leak somewhere but I can't seem to figure it out. I've tried deleting the instance at the end of the function but that doesn't help.
void drawCircle(cairo_surface_t *container, int x, int y, int radius, float r, float g, float b, float a)
{
cairo_t *cairoInstance;
cairoInstance = cairo_create(container);
cairo_set_source_rgba(cairoInstance, r, g, b, a);
cairo_arc(cairoInstance, x, y, radius, 0, 2*M_PI);
cairo_stroke_preserve(cairoInstance);
cairo_fill_preserve(cairoInstance);
//delete cairoInstance;
gtk_widget_queue_draw_area(GTK_WIDGET(frame2), 0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
}
Any ideas?
Thanks in advance.

Answer 1

A few points:
cairo_ts are reference counted. Call cairo_destroy when you are done with your cairo_t*. Alternatively, you don't need to create and destroy cairo_t's for each circle - refactor the function by pulling out the call to cairo_create.
Unless you need them, prefer the cairo_X functions to the cairo_X_preserve ones. In your code cairo_fill_preserve should probably be cairo_preserve. (The stroke should be preserved though, so that the following fill works.)
The call to invalidate a rectangle of your gtk widget with gtk_widget_queue_draw_area can be refactored out as well, and only done once per draw.
gtk_widget_queue_draw_area invalidates a rectangular region of the widgets window - you may be fine with just gdk_window_invalidate_rect - see the documentation
Psuedo-code after refactoring (with a hypothetical Circle type):
void drawCircle(cairo_t *cr, int x, int y, int radius, float r, float g, float b, float a) {
cairo_set_source_rgba(cr, r, g, b, a);
cairo_arc(cr, x, y, radius, 0, 2*M_PI);
cairo_stroke_preserve(cr); // keep the arc so that we can call cairo_fill
cairo_fill(cr);
}
void functionThatDrawsCircles(cairo_surface_t* surface, Circle* circles, int num) {
cairo_t* cr = cairo_create(surface);
for(int i = 0; i < num; i++) {
drawCircle(cr, circles[i].x, circles[i].y, 10, circles[i].r, circles[i].g, circles[i].b, 1.0);
}
cairo_destroy(cr);
gtk_widget_queue_draw_area(GTK_WIDGET(frame2), 0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
}