void string_replace(){
  std::string s = "a, b, c";     // 置換対象の文字列
  std::string target = ", ";     // 検索文字列
  std::string replacement = "-"; // 置換文字列
  cout<<s<<endl;
  if (!target.empty()) {
    std::string::size_type pos = 0;
    while ((pos = s.find(target, pos)) != std::string::npos) {
      s.replace(pos, target.length(), replacement);
      pos += replacement.length();
    }
  }
  cout<<s<<endl;
return;
}

===================================
a, b, c
a-b-c