my window disappears quickly

[Zlatomir]

That happens because you allocate memory (for your g) on that function stack.

You should allocate memory on the heap, so the object remains after the function terminates, like this:

drawGraph *g = new drawGraph; //create a pointer and allocate with new
g->show(); // use  -> to access members

drawGraph *g = new drawGraph; //create a pointer and allocate with new
g->show(); // use  -> to access members

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And make sure that your g pointer gets a parent, or else you should call delete g; in the class destructor (so that you wont get a memory leak)