c++ - What are the syntax rules for ## in define?

Question

In allocation.hpp of the jdk8 source code, there is

#define METASPACE_OBJ_TYPE_DECLARE(name) name ## Type,
#define METASPACE_OBJ_TYPE_NAME_CASE(name) case name ## Type: return #name;

  enum Type {
    // Types are MetaspaceObj::ClassType, MetaspaceObj::SymbolType, etc
    METASPACE_OBJ_TYPES_DO(METASPACE_OBJ_TYPE_DECLARE)
    _number_of_types
  };

I don’t understand what grammatical rules ## are in define.

Answer 1

#define is to define variables, the two ## are connection operators, you can view

Answer 2

## is the concatenation operator, see https://stackoverflow.com/que...