how to delete a static object in c++ - c++

Question

I'm trying to build simple painter (i.e. points, lines, circles ...etc) in glut. Each line must have two points of Point type, so every time the user enter the left button of the mouse, the chosen command is executed. For drawing a line, I need to track how many times the user click the mouse, so this is what I'v done
if ( command == 1 ){ // drawing a line
static int count(0); // track click no.
static std::vector<Point> p;
//static Point startPoint(mouseX, mouseY);
p.push_back(Point(mouseX, mouseY));
if ( count == 1 ){
Point endPoint(mouseX, mouseY);
Point startPoint = p[0];
shapes->addLine(Line(startPoint, endPoint));
count = 0;
p.clear();
}else{
count++;
}
I'm using std::vector only to use clear() so that I can delete startPoint which I need it to be static. My question is is there a way to destroy an object without making more lines by using vector? I've tried to call the destructor but it didn't help.

Answer 1

You could use a unique_ptr<Point>. Then you can use reset to set or destroy the Point:
static std::unique_ptr<Point> startPoint;
if (startPoint){
Point endPoint(mouseX, mouseY);
shapes->addLine({*startPoint, endPoint});
startPoint.reset();
} else {
startPoint.reset(new Point(mouseX, mouseY));
}

Answer 2

Your code is fine. If you're worried about the number of lines then this is a shorter version:
if ( command == 1 ){ // drawing a line
static std::vector<Point> p;
p.push_back(Point(mouseX, mouseY));
if (p.size() == 2){
shapes->addLine(Line(p[0], p[1]));
p.clear();
}
}
Note however that using less lines is only a good thing if this improves readability. If instead it becomes harder to understand the code then it's a bad idea.
Most code is written only once but read many times... saving time when writing is not such a big deal.
In this specific case in my opinion this shorter version is easier to understand, but your mileage may vary.

Answer 3

This is one of those times where something like std::optional<Point> would've been nice.
But regarding to the destruction and reconstruction part, placement new can be helpful here:
static int count(0);
// ('aligned_storage' requires C++11 and '#include <type_traits>')
static std::aligned_storage<sizeof(Point), alignof(Point)>::type startPointBuffer;
Point& startPoint = *static_cast<Point*>(static_cast<void*>(&startPointBuffer));
if (count == 1) {
Point endPoint(mouseX, mouseY);
shapes->addLine(Line(startPoint, endPoint));
count = 0;
startPoint.~Point();
} else {
new (&startPoint) Point(mouseX, mouseY);
count++;
}