Some refactorings:

- Overrides added
 - usages of push_back() replaced with emplace_back()
 - Loop variable made const-refernce
 - NULL replaced with nullptr
 - Names used in the declaration and definition unified
 - size() replaced with empty
 - Identical cases merged

examples/231-Cfg-OutputStreams.cpp

@@ -18,7 +18,7 @@ class out_buff : public std::stringbuf {
 public:
     out_buff(std::FILE* stream):m_stream(stream) {}
     ~out_buff();
-    int sync() {
+    int sync() override {
         int ret = 0;
         for (unsigned char c : str()) {
             if (putc(c, m_stream) == EOF) {