C++ static function which will replace envionment variable name denoted by a prefix and corresponding postfix

ResolveEnvVars.h

// Replace environment variables identified by a prefix and postfix string with their corresponding values
// Returns a new modified string or the original string if it encounters incorrect formatted input
static std::string ResolveEnvVars(const std::string& str, const std::string prefix, const std::string postfix)
{
   size_t p = 0;
   size_t a = std::string::npos;
   size_t b = std::string::npos;
   std::string outstr = str;
   bool found = true;
   bool error = false;

   while (found)
   {
	  found = false;
	  a = outstr.find(prefix, p);
	  if (a != std::string::npos)
	  {
		 b = outstr.find(postfix, a + prefix.length());
		 if (b != std::string::npos)
		 {
			std::string varname = outstr.substr(a + prefix.length(), b - a - prefix.length());
			char* varvalue = std::getenv(varname.c_str());

			if (varvalue != NULL)
			{
			   outstr.replace(a, b - a + postfix.length(), varvalue);
			   p += a + std::strlen(varvalue);
			   found = true;
			}
			else error = true;
		 }
		 else error = true;
	  }
   }

   return error ? str : outstr;
}