Which webMethods product are you asking about?
Assuming that you’re interested in measuring the productivity of a develpoer using webMethods IS Developer, are you talking about exposing existing application functionality so that it can be accessed as a web service or using IS to consume some already existing web service provided by another application or entity?
Additionally, against what are you trying to measure productivity? Are you suggesting that someone is considering consuming web services using C or C++? One could certainly do so, but I’m not sure the advantages of those languages in terms of speed of execution would make sense given the services being consumed or provided would mostly likely be invoked using SOAP over HTTP/S.