Redefine ByteBufferWriterT template to really use argument

Previously, ByteBufferWriterT could only be instantiated for "char",
due to using "char" internally. The rewrite extracts the inner type
from the BufferT parameter and uses that.

This is preparatory to changing its type to "uint8_t".

Bug: webrtc:15665
Change-Id: Ib771d37e3abb8261049c16122c6b43dcb561e9a0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/329680
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41306}

rtc_base/byte_buffer.h

@@ -27,39 +27,43 @@ namespace rtc {
 
 template <class BufferClassT>
 class ByteBufferWriterT {
+  using value_type = typename BufferClassT::value_type;
+
  public:
   ByteBufferWriterT() { Construct(nullptr, kDefaultCapacity); }
-  ByteBufferWriterT(const char* bytes, size_t len) { Construct(bytes, len); }
+  ByteBufferWriterT(const value_type* bytes, size_t len) {
+    Construct(bytes, len);
+  }
 
   ByteBufferWriterT(const ByteBufferWriterT&) = delete;
   ByteBufferWriterT& operator=(const ByteBufferWriterT&) = delete;
 
-  const char* Data() const { return buffer_.data(); }
+  const value_type* Data() const { return buffer_.data(); }
   size_t Length() const { return buffer_.size(); }
   size_t Capacity() const { return buffer_.capacity(); }
 
   // Write value to the buffer. Resizes the buffer when it is
   // neccessary.
   void WriteUInt8(uint8_t val) {
-    WriteBytes(reinterpret_cast<const char*>(&val), 1);
+    WriteBytes(reinterpret_cast<const value_type*>(&val), 1);
   }
   void WriteUInt16(uint16_t val) {
     uint16_t v = HostToNetwork16(val);
-    WriteBytes(reinterpret_cast<const char*>(&v), 2);
+    WriteBytes(reinterpret_cast<const value_type*>(&v), 2);
   }
   void WriteUInt24(uint32_t val) {
     uint32_t v = HostToNetwork32(val);
-    char* start = reinterpret_cast<char*>(&v);
+    value_type* start = reinterpret_cast<value_type*>(&v);
     ++start;
     WriteBytes(start, 3);
   }
   void WriteUInt32(uint32_t val) {
     uint32_t v = HostToNetwork32(val);
-    WriteBytes(reinterpret_cast<const char*>(&v), 4);
+    WriteBytes(reinterpret_cast<const value_type*>(&v), 4);
   }
   void WriteUInt64(uint64_t val) {
     uint64_t v = HostToNetwork64(val);
-    WriteBytes(reinterpret_cast<const char*>(&v), 8);
+    WriteBytes(reinterpret_cast<const value_type*>(&v), 8);
   }
   // Serializes an unsigned varint in the format described by
   // https://developers.google.com/protocol-buffers/docs/encoding#varints
@@ -68,22 +72,24 @@ class ByteBufferWriterT {
     while (val >= 0x80) {
       // Write 7 bits at a time, then set the msb to a continuation byte
       // (msb=1).
-      char byte = static_cast<char>(val) | 0x80;
+      value_type byte = static_cast<value_type>(val) | 0x80;
       WriteBytes(&byte, 1);
       val >>= 7;
     }
-    char last_byte = static_cast<char>(val);
+    value_type last_byte = static_cast<value_type>(val);
     WriteBytes(&last_byte, 1);
   }
   void WriteString(absl::string_view val) {
     WriteBytes(val.data(), val.size());
   }
-  void WriteBytes(const char* val, size_t len) { buffer_.AppendData(val, len); }
+  void WriteBytes(const value_type* val, size_t len) {
+    buffer_.AppendData(val, len);
+  }
 
-  // Reserves the given number of bytes and returns a char* that can be written
-  // into. Useful for functions that require a char* buffer and not a
-  // ByteBufferWriter.
-  char* ReserveWriteBuffer(size_t len) {
+  // Reserves the given number of bytes and returns a value_type* that can be
+  // written into. Useful for functions that require a value_type* buffer and
+  // not a ByteBufferWriter.
+  value_type* ReserveWriteBuffer(size_t len) {
     buffer_.SetSize(buffer_.size() + len);
     return buffer_.data();
   }
@@ -97,7 +103,7 @@ class ByteBufferWriterT {
  private:
   static constexpr size_t kDefaultCapacity = 4096;
 
-  void Construct(const char* bytes, size_t size) {
+  void Construct(const value_type* bytes, size_t size) {
     if (bytes) {
       buffer_.AppendData(bytes, size);
     } else {