The disclosed embodiments provide a system for densely storing strings within the memory of a software program. During operation, the system receives a set of characters to be stored in a string, wherein each character is encoded using a multi-byte public encoding. The system then stores the set of characters in the string at least in part by performing the following steps. The system determines whether every character of the set can be encoded using a first private encoding that consumes less space per character than the multi-byte public encoding. Responsive to determining that every character of the set can be encoded using the first private encoding, the system stores a particular value associated with the first private encoding in a particular field of the string. The system then stores the set of characters in the string in the first private encoding.