from The Teaching of the Catholic Church, Volume II, Canon George D. Smith:

First, Has Christ instituted any other positive means of regeneration besides baptism, either by way of addition to or exception from the law of baptism? Secondly, Is it not possible that, from the very nature of things which precedes all positive law and is allowed for in positive law, it might happen that a person could receive justification without the actual reception of the Sacrament of Baptism?
 
We answer to the first of these questions in the negative. We cannot admit any other means of salvation positively instituted by Christ, for the very good reason that his positive law has provided one means and only one. If, therefore, any theories are advanced on the question of salvation which involves the recognition of some means of salvation positively instituted by Christ, other than baptism, such theories must immediately be rejected as at least erroneous.
 
We answer to the second question in the affirmative. It can happen that a person receives justification without actually receiving the Sacrament of Baptism. And it can happen in one of two ways: either, 1. by Martyrdom, or 2. by Charity.
 
p.778-779
 
…. Now, an act of charity always and necessarily contains a desire for the Sacrament of Baptism, hence the expression Baptism of Desire. The reason why it must contain this desire is that an act of the love of God must contain a desire of conforming to his will in every way. Therefore, since it is God’s will that we should receive the Sacrament of Baptism, this act must contain the desire for baptism. But this desire may either be implicitly or explicitly, and each alternative requires our careful consideration.
 
It is implicit in anyone who makes an act of the love of God, and, through invincible ignorance, does not know of the necessity of sacrament baptism. This might happen in a country like England to people who are not baptised.
 
Might it not also happen to heathens who have never heard of Christ?
 
It might, if we suppose that these heathens have in some way obtained the necessary minimum knowledge of Revelation, and are capable of a salutary faith and hope in God. For it is very important to understand that when we speak of charity, we do not mean just any kind of love of God above all else, such as the natural love of a creature for its Creator. Charity is essentially a love of friendship (Our Blessed Lord does not call us servants, but friends), which implies an intimate communication with God, such as is only possible in a supernatural order. The existence of this supernatural order can only be known through Revelation. Charity, therefore, cannot exist without at least the knowledge of the principle truth of Revelation. p. 782-783