when org-src-font-lock-fontify-block copies text from the buffer named "
org-src-fontification:c++-mode" to code block,
#+BEGIN_SRC c++
  void foo()
  {
  }
#+END_SRC
it copies face text properties of strings at the same time.
However, face text property relies on the default face of the source buffer.
If the default face of the source buffer and the default face of the target
buffer is different,
text in source blocks will have different appearances than the source buffer
depending on the default faces.

Also, if  I use buffer-face-set in the org-mode buffer, text in code block
will change too.

I think it is better to incorporate the default face of c++-mode buffer to
the face of text in code block in
org-mode buffer when copy string using org-src-font-lock-fontify-block.