Parameter pack expansion within parentheses gives bizarre output

I am trying to implement a function which accepts a variable number of strings and forwards to a print function, which expects a char pointer and size for every string, interleaved.

Example:

std::string a = "123";
std::string b = "1234";
forward(a, b); // should call doPrint(a.c_str(), a.size(), b.c_str(), b.size())

I thought that the following should be a correct implementation, but even though it compiles the behavior is very surprising to me.

template <class ...Args>
void forward(const Args & ... args) {
  doPrint( (args.c_str(), args.size())...);
}

forward(a, b) calls doPrint(3, 4), and not doPrint("123", 3, "1234", 4), as if I had written doPrint((args.size())...). The call to c_str() is ignored completely by the compiler.

I tried g++, clang, and icc with all yielding the same output. What is wrong with (args.c_str(), args.size())...?

Indeed, std::make_tuple(args.c_str(), args.size())... works as expected, but let's say I cannot change doPrint to accept and process tuples.