Two of the most popular libraries for SOAP are Apache Axis and gSOAP.
Apache Axis is suitable for implementing both SOAP (including WSDL) and REST web services. It's worth noting that the library hasn't seen a new release for over a decade.
gSOAP is a toolkit that allows for creating and interacting with XML-based web services with a focus on SOAP. It handles data binding, SOAP and WSDL support, JSON and RSS parsing, UDDI APIs, and several other related web services standards. Although it doesn't use modern C++ features, it is still actively maintained.