One embodiment of the present invention provides a system for implementing a string object defined in a programming language. Upon receiving characters to be embedded in the string object, the system allocates space for the string object that includes a header, which is appended to a character array of the string object. Next, the system copies the characters into the character array of the string object. The system also initializes the header by initializing a class pointer in the header to point to a class for the string object, and by initializing a length field in the header to specify a length for the string object. In this way, the string object remains compatible with previous implementations of the string object that use a character array that is separate from the string object.