Charles was an heir presumptive from his birth to the death of his
grandfather, as he could have had an uncle while his grandfather, George VI,
was alive.
IIRC, it was posted somewhere in the newsgroup that George VI and Queen
Elizabeth decided not to have any more children some time after Princess
Margaret was born.
Obviously, Queen Elizabeth was 48 years old at the time of Charles's
birth, and rather unlikely to even be capable of having children.
But, of course, she might die, and the king remarry, and have a son.
As long as it's conceivably possible that another heir might be born,
you don't have an heir apparent. In fact, I think there are some
circumstances where it would not even conceivably be possible for the
presumptive heir to be displaced, but yet they are not heir apparent.
For instance, let's say all of Queen Victoria's sons die in a boiler
explosion in 1862 (silly, I know). The Queen does not remarry. In
1879, she is 60 years old, and obviously incapable of having any more
children. Still, Vicky would not be the heiress-apparent, although,
in actual fact, there is no physically possible way that anybody could
displace her. (This only really works with queens regnant, I'd add,
since with a King it's always conceivable that they might be able to
father a child)
a) HM George VI
b) HRH the Princess Elizabeth
c) HRH Prince Charles (LP 1948)
d) HRH Princess Anne (LP 1948/b. 1950)
e) HRH the Princess Margaret
f) HRH the Duke of Gloucester + family
g) HRH the Duke of Kent + family
no family
h) HRH Prince Michael of Kent
i) HRH Princess Alexandra of Kent
Succession at the end of 1950:
HM George VI
1. HRH The Duchess of Edinburgh (heiress presumptive)
2. HRH Prince Charles of Edinburgh
3. HRH Princess Anne of Edinburgh
4. HRH the Princess Margaret
5. HRH The Duke of Gloucester
6. HRH Prince William of Gloucester
7. HRH Prince Richard of Gloucester (now the Duke of Gloucester)
8. HRH the Duke of Kent
9. HRH Prince Michael of Kent
10. HRH Princess Alexandra of Kent
11. HRH the Princess Royal
12. The Earl of Harewood
13. Viscount Lascelles (b.1950)
14. Hon. Gerald Lascelles
15. HRH Princess Arthur of Connaught (AKA the suo jure Duchess of
Fife)
16. Lord Carnegie (now the Duke of Fife)
17. HRH Crown Prince Olav of Norway
18. HRH Hereditary Prince Harald of Norway (the present King)
19. HRH Princess Ragnhild of Norway
20. HRH Princess Astrid of Norway
Wow, exactly 20 descendants of Edward VII. Who'd have thought? And
only one of these (Gerald Lascelles) wholly untitled, although
Viscount Lascelles and Lord Carnegie did not possess substantive
titles.