docs: address community issue feedback (#5 #338 #142 #246 #136 #303)

- const README: rewrite the space-saving claim per issue #5 follow-ups
  (constant-expression const vs #define immediates; which const qualify
  as constant expressions)
- const README: fix the class-section diagnosis per issue #338 (overload
  resolution picks the const overload; the restriction only surfaces
  once that overload is removed)
- sizeof README: virtual-inheritance result is compiler-dependent
  (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246
- static README: new section on internal linkage / access restriction
  with anonymous-namespace comparison #142
- 10-day practice: full problem list table + fixed day links #136
- new design_pattern/singleton/singleton_default.hpp: reusable
  thread-safe CRTP singleton template, compile+concurrency tested #303

site_extras/cpp-projects-protobuf-zerocopy-cpp.md

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+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-projects/protobuf-zerocopy-cpp/) 的文章，由站点同步脚本导出。
+
+# Protobuf ZeroCopy 序列化
+
+## 1.ZeroCopyStream
+
+protobuf在io接口上有一个叫做ZeroCopyStream，对于IO的接口设计，pb提供了相关序列化与反序列化接口，如：
+
+```cpp
+// Read a protocol buffer from the given zero-copy input stream.  If
+// successful, the entire input will be consumed.
+bool ParseFromZeroCopyStream(io::ZeroCopyInputStream* input);
+
+// Write the message to the given zero-copy output stream.  All required
+// fields must be set.
+bool SerializeToZeroCopyStream(io::ZeroCopyOutputStream* output) const;
+```
+
+ZeroCopyStream设计的初衷是最小化buffer的拷贝次数，即省略掉stream内部数据拷贝到用户buffer。因此，stream可以返回一个缓冲区，该缓冲区实际上直接指向要存储字节的最终数据结构，并且调用者可以直接与该缓冲区交互，从而消除了中间复制操作。
+
+例如：经典的IO stream：
+
+```cpp
+char buffer[[]BUFFER_SIZE];
+input->Read(buffer, BUFFER_SIZE);
+DoSomething(buffer, BUFFER_SIZE);
+```
+
+然后，stream基本上只是调用memcpy()将数据从pb复制到用户buffer中。如果使用ZeroCopyInputStream，我们只需要：
+
+```cpp
+const void* buffer;
+int size;
+input->Next(&buffer, &size);
+DoSomething(buffer, size);
+```
+
+这里不执行拷贝，调用者最终直接从buffer中读取。
+
+ZeroCopyStream提供了两个基类：ZeroCopyOutputStream/ZeroCopyInputStream。
+
+以InputStream为例，通常我们可以通过继承的方式自定义自己的ZerCopyStream，需要实现下面四个接口。
+
+```cpp
+// implements ZeroCopyInputStream ----------------------------------
+bool Next(const void** data, int* size);
+void BackUp(int count);
+bool Skip(int count);
+int64 ByteCount() const;
+```
+
+一些rpc框架基本都自定义自己的Stream，例如：sofa-pbrpc
+
+> https://github.com/baidu/sofa-pbrpc/blob/master/src/sofa/pbrpc/buffer.h
+
+## 2.Demo
+
+定义一个pb协议，例如：授权验证：
+
+```cpp
+syntax = "proto3";
+
+message BasicAuth {
+  string username = 1;
+  string password = 2;
+}
+```
+
+随后编写序列化与反序列化：
+
+- 序列化
+
+```cpp
+BasicAuth auth_message;
+auth_message.set_username("user123");
+auth_message.set_password("password");
+
+StringOutputStream output_stream(&buf);
+
+auth_message.SerializeToZeroCopyStream(&output_stream);
+```
+
+- 反序列化
+
+```cpp
+ArrayInputStream input_stream(buf.data(), buf.size());
+BasicAuth auth_message;
+if (!auth_message.ParseFromZeroCopyStream(&input_stream)) {
+    std::cerr << "Failed to parse data." << std::endl;
+}
+
+std::cout << "Username: " << auth_message.username() << std::endl;
+std::cout << "Password: " << auth_message.password() << std::endl;
+```
+
+随后我们便可以调用它：
+
+```cpp
+int main() {
+    std::string buf;
+    buf.reserve(512);
+    ser(buf);
+    deser(buf);
+    return 0;
+}
+```
+
+输出：
+
+```cpp
+Username: user123
+Password: password
+```
+
+本节完，以上完整代码见知识星球，欢迎与我共同探讨rpc相关内容。
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+(gdb) p	*dsr
+$8 = {typeInfo = {datumlen = 4, typid = 1082, align = 105 'i', byval = true}, datumStreamVersion = DatumStreamVersion_Original, nth = -1,
+  logical_row_count = 16382, datump = 0x3840470 "\277\377\377\377\231\002", physical_datum_index = -1, physical_datum_count = 0,
+  datum_beginp = 0x3840470 "\277\377\377\377\231\002", datum_afterp = 0x38503dc "", has_null = true, buffer_beginp = 0x383fc60 "",
+  null_bitmap = {buffer = 0x383fc70 "", byteBit = 0 '\000', bytePointer = 0x383fc70 "", bitCount = 16382, bitPosition = -1,
+    readBitOnCount = 58981488}, null_bitmap_beginp = 0x0, rle_can_have_compression = false, rle_compress_beginp = 0x0, rle_repeatcountsp = 0x0,
+  delta_beginp = 0x0, delta_deltasp = 0x0, delta_datum_p = 0, rle_norepeats_null_bitmap_count = 0, rle_compress_bitmap_count = 0,
+  rle_repeatcounts_count = 0, rle_repeatcounts_size = 0, delta_item = false, delta_bitmap_count = 0, deltas_count = 0, deltas_size = 0,
+  rle_block_was_compressed = false, rle_compress_bitmap = {buffer = 0x0, byteBit = 0 '\000', bytePointer = 0x0, bitCount = 0, bitPosition = 0,
+    readBitOnCount = 0}, rle_repeatcounts_index = 0, rle_in_repeated_item = false, rle_repeated_item_count = 0,
+  rle_total_repeat_items_read = 0, delta_block_was_compressed = false, delta_bitmap = {buffer = 0x0, byteBit = 0 '\000',
+    bytePointer = 0xff6c00454244 <error: Cannot access memory at address 0xff6c00454244>, bitCount = 0, bitPosition = 0,
+    readBitOnCount = 13651600}, eyecatcher = "\000\000\000", physical_data_size = 0, maxDataBlockSize = 13652112,
+  errdetailCallback = 0x35daeb0, errdetailArg = 0x34c4e18, errcontextCallback = 0x34c4e18, errcontextArg = 0x383fc60, memctxt = 0x1077c}
+```