Add 'ostream' method to class StreamBuffer, and use it.

luprex/core/cpp/streambuffer.hpp

@@ -239,7 +239,7 @@ public:
 };
 
 class StreamBuffer {
-public:
+public:    
     // Construct an empty buffer.
     StreamBuffer();
 
@@ -387,6 +387,9 @@ public:
     static const char *lua_reader(lua_State *L, void *data, size_t *size);
     void *lua_reader_ud(int64_t bytes);
 
+    // Get an ostream that writes into the StreamBuffer.
+    std::ostream &ostream();
+
 private:
     // Start and end of the allocated block.
     char *buf_lo_;
@@ -409,6 +412,9 @@ private:
     const char *lua_reader_data_;
     int64_t lua_reader_size_;
 
+    // The ostream.  Only allocated on demand.
+    std::unique_ptr<std::ostream> ostream_;
+
     // Initialize with a new buffer.
     void init(bool fixed, bool owned, char *buf, int64_t size);