Since XML-RPC is no longer actively maintained, there aren't any modern C++ implementations for the standard. If you want to interact with XML-RPC web services from modern code, the best way may be to use the gSOAP toolkit that supports XML-RPC and other XML web service standards.
The main criticism of XML-RPC was that it didn't give much value over sending plain XML requests and responses while making the messages significantly larger.
As the standard evolved, it became SOAP. As SOAP it formed the basis for the W3C web services stack of protocols.