• Mark the stringstream as public. This won't affect most uses of your code, and can already be hacked around with a custom manipulator to get at the "internal" stream object, but it will enable those that need to access the internal stream (such as to avoid the string copy inherent in the stringstream interface) to do so. Of course, 0x move semantics allay much of this need, but are still Not Quite Here Yet™.

• Check the stream before returning the string; if it's in a failed state, throw an exception (or at least log the condition somewhere before returning a string). This is unlikely to occur for most uses, but if it does happen, you'll be glad you found out the stream is failed rather than screw with formatting while wondering why "it just won't work".

• Mark the stringstream as public. This won't affect most uses of your code, and can already be hacked around with a custom manipulator to get at the "internal" stream object, but it will enable those that need to access the internal stream (such as to avoid the string copy inherent in the stringstream interface), and know the specifics of their implementation that allow what they want, to do so. Of course, 0x move semantics allay much of this need, but are still Not Quite Here Yet™.

• Check the stream before returning the string; if it's in a failed state, throw an exception (or at least log the condition somewhere before returning a string). This is unlikely to occur for most uses, but if it does happen, you'll be glad you found out the stream is failed rather than screw with formatting while wondering why "it just won't work".

• Formatter() and expr_a both occur before the insertion marked 1.
• The above, plus insertion 1, plus expr_b happen before insertion 2.
• The above, plus insertion 2, plux expr_c happen before insertion 3.
• Note this only limits in one direction: expr_c can happen after expr_a and before Formatter(), for example.
• Naturally, all of the above plus the string conversion occur before calling some_function.

• Formatter() and expr_a both occur before the insertion marked 1.
• The above, plus insertion 1, plus expr_b happen before insertion 2.
• The above, plus insertion 2, plus expr_c happen before insertion 3.
• Note this only limits in one direction: expr_c can happen after expr_a and before Formatter(), for example.
• Naturally, all of the above plus the string conversion occur before calling some_function.

In order to handle all manipulators, such as std::endl, add:

struct Formatter {
  Formatter& operator<<(std::ios_base& (*manip)(std::ios_base&)) {
    ss_ << manip;
    return *this;
  }
  Formatter& operator<<(std::ios& (*manip)(std::ios&)) {
    ss_ << manip;
    return *this;
  }
  Formatter& operator<<(std::ostream& (*manip)(std::ostream&)) {
    ss_ << manip;
    return *this;
  }
};