CTS Mode or CBC-CTS is ciphertext stealing mode.
Crypto++ does not provide a way to retrieve the current IV or counter used for encryption or decryption. If you need the current IV or counter then you need to manage it yourself. Some ciphers allow you to seek a number of bytes or blocks in the stream.
If you are used to working in languages like Jave or libraries like OpenSSL, then you might want to visit the Init-Update-Final wiki page. Crypto++ provides the transformation model, but its not obvious because its often shrouded behind Pipelines.
Note: if your project is using encryption alone to secure your data, encryption alone is usually not enough. Please take a moment to read Authenticated Encryption and consider using an algorithm or mode like CCM, GCM, EAX or ChaCha20Poly1305.