In Macbeth, who is the heir to the throne of Scotland?

In Act 1, Scene 4, Duncan announces that his older son, Malcolm, is named the Prince of Cumberland, which is the title for the heir to the throne (I.4.45). Therefore, Malcolm is the official heir, the person who is supposed to be the king after Duncan. Macbeth was told by the Weird Sisters in Scene 3, however, that he would king "hereafter" (1.3.53).  The witches are not specific about when Macbeth will become the monarch, but because he learns that he's been named the new Thane of Cawdor almost immediately after the witches tell him he will be, Macbeth seems to hope and expect that the kingship will also come to him quickly and easily. He is quite disappointed when Duncan announces Malcolm as Prince of Cumberland. Macbeth says to himself, "That is a step / On which I must fall down or else o'erleap" (I.4.55-56). In other words, because Macbeth has not been named Duncan's heir, he must either curb his ambition for the throne or somehow overcome that obstacle to becoming king. Ultimately, he chooses the latter.