Turning temporary stringstream to c_str() in single statement

You cannot cast the temporary stream to std::ostringstream&. It is ill-formed (the compiler must tell you that it is wrong). The following can do it, though:

f(static_cast<std::ostringstream&>(
  std::ostringstream().seekp(0) << "Value: " << 5).str().c_str());

That of course is ugly. But it shows how it can work. seekp is a member function returning a std::ostream&. Would probably better to write this generally

template<typename T>
struct lval { T t; T &getlval() { return t; } };

f(static_cast<std::ostringstream&>(
  lval<std::ostringstream>().getlval() << "Value: " << 5).str().c_str());

The reason that without anything it takes the void*, is because that operator<< is a member-function. The operator<< that takes a char const* is not.

A temporary cannot be passed as a non-const reference to a function, that's why it does not find the correct streaming operator and instead takes the one with void* argument (it is a member function and thus calling it on a temporary is OK).

What comes to getting around the limitations by casting, I have a feeling that it is in fact UB, but I cannot say for sure. Someone else will surely quote the standard.