c++ - 類別的成員看著像函數，但卻長著一張陣列的臉，它究竟是什麼？

Question

// This class is a friend of most classes, to be able to access
// private fields
class VMStructs {
public:
  // The last entry is identified over in the serviceability agent by
  // the fact that it has a NULL fieldName
  static VMStructEntry localHotSpotVMStructs[];

  // The last entry is identified over in the serviceability agent by
  // the fact that it has a NULL typeName
  static VMTypeEntry   localHotSpotVMTypes[];

  // Table of integer constants required by the serviceability agent.
  // The last entry is identified over in the serviceability agent by
  // the fact that it has a NULL typeName
  static VMIntConstantEntry localHotSpotVMIntConstants[];

  // Table of long constants required by the serviceability agent.
  // The last entry is identified over in the serviceability agent by
  // the fact that it has a NULL typeName
  static VMLongConstantEntry localHotSpotVMLongConstants[];

  // This is used to run any checking code necessary for validation of
  // the data structure (debug build only)
  static void init();

#ifndef PRODUCT
  // Execute unit tests
  static void test();
#endif

private:
  // Look up a type in localHotSpotVMTypes using strcmp() (debug build only).
  // Returns 1 if found, 0 if not.
  //  debug_only(static int findType(const char* typeName);)
  static int findType(const char* typeName);
};

其中我看不懂的地方就是

• static VMStructEntry localHotSpotVMStructs[];

• static VMTypeEntry localHotSpotVMTypes[];
  #...

#這樣的定義究竟是什麼呢？看著像函數，但卻又用的是數組，它究竟是什麼呢？

其中，localHotSpotVMStructs[] 定義如下

// These initializers are allowed to access private fields in classes
// as long as class VMStructs is a friend
VMStructEntry VMStructs::localHotSpotVMStructs[] = {

  VM_STRUCTS(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_STATIC_VM_STRUCT_ENTRY,
             GENERATE_UNCHECKED_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_NONPRODUCT_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_C1_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_C2_NONSTATIC_VM_STRUCT_ENTRY,
             GENERATE_C1_UNCHECKED_STATIC_VM_STRUCT_ENTRY,
             GENERATE_C2_UNCHECKED_STATIC_VM_STRUCT_ENTRY)

#if INCLUDE_ALL_GCS
  VM_STRUCTS_PARALLELGC(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                        GENERATE_STATIC_VM_STRUCT_ENTRY)

  VM_STRUCTS_CMS(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_STATIC_VM_STRUCT_ENTRY)

  VM_STRUCTS_G1(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                GENERATE_STATIC_VM_STRUCT_ENTRY)
#endif // INCLUDE_ALL_GCS

#if INCLUDE_TRACE
  VM_STRUCTS_TRACE(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                GENERATE_STATIC_VM_STRUCT_ENTRY)
#endif

  VM_STRUCTS_CPU(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_STATIC_VM_STRUCT_ENTRY,
                 GENERATE_UNCHECKED_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_NONPRODUCT_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_C2_NONSTATIC_VM_STRUCT_ENTRY,
                 GENERATE_C1_UNCHECKED_STATIC_VM_STRUCT_ENTRY,
                 GENERATE_C2_UNCHECKED_STATIC_VM_STRUCT_ENTRY)

  VM_STRUCTS_OS_CPU(GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                    GENERATE_STATIC_VM_STRUCT_ENTRY,
                    GENERATE_UNCHECKED_NONSTATIC_VM_STRUCT_ENTRY,
                    GENERATE_NONSTATIC_VM_STRUCT_ENTRY,
                    GENERATE_NONPRODUCT_NONSTATIC_VM_STRUCT_ENTRY,
                    GENERATE_C2_NONSTATIC_VM_STRUCT_ENTRY,
                    GENERATE_C1_UNCHECKED_STATIC_VM_STRUCT_ENTRY,
                    GENERATE_C2_UNCHECKED_STATIC_VM_STRUCT_ENTRY)

  GENERATE_VM_STRUCT_LAST_ENTRY()
};

localHotSpotVMTypes[]的定義如下

VMTypeEntry VMStructs::localHotSpotVMTypes[] = {

  VM_TYPES(GENERATE_VM_TYPE_ENTRY,
           GENERATE_TOPLEVEL_VM_TYPE_ENTRY,
           GENERATE_OOP_VM_TYPE_ENTRY,
           GENERATE_INTEGER_VM_TYPE_ENTRY,
           GENERATE_UNSIGNED_INTEGER_VM_TYPE_ENTRY,
           GENERATE_C1_TOPLEVEL_VM_TYPE_ENTRY,
           GENERATE_C2_VM_TYPE_ENTRY,
           GENERATE_C2_TOPLEVEL_VM_TYPE_ENTRY)

#if INCLUDE_ALL_GCS
  VM_TYPES_PARALLELGC(GENERATE_VM_TYPE_ENTRY,
                      GENERATE_TOPLEVEL_VM_TYPE_ENTRY)

  VM_TYPES_CMS(GENERATE_VM_TYPE_ENTRY,
               GENERATE_TOPLEVEL_VM_TYPE_ENTRY)

  VM_TYPES_PARNEW(GENERATE_VM_TYPE_ENTRY)

  VM_TYPES_G1(GENERATE_VM_TYPE_ENTRY,
              GENERATE_TOPLEVEL_VM_TYPE_ENTRY)
#endif // INCLUDE_ALL_GCS

#if INCLUDE_TRACE
  VM_TYPES_TRACE(GENERATE_VM_TYPE_ENTRY,
              GENERATE_TOPLEVEL_VM_TYPE_ENTRY)
#endif

  VM_TYPES_CPU(GENERATE_VM_TYPE_ENTRY,
               GENERATE_TOPLEVEL_VM_TYPE_ENTRY,
               GENERATE_OOP_VM_TYPE_ENTRY,
               GENERATE_INTEGER_VM_TYPE_ENTRY,
               GENERATE_UNSIGNED_INTEGER_VM_TYPE_ENTRY,
               GENERATE_C1_TOPLEVEL_VM_TYPE_ENTRY,
               GENERATE_C2_VM_TYPE_ENTRY,
               GENERATE_C2_TOPLEVEL_VM_TYPE_ENTRY)

  VM_TYPES_OS_CPU(GENERATE_VM_TYPE_ENTRY,
                  GENERATE_TOPLEVEL_VM_TYPE_ENTRY,
                  GENERATE_OOP_VM_TYPE_ENTRY,
                  GENERATE_INTEGER_VM_TYPE_ENTRY,
                  GENERATE_UNSIGNED_INTEGER_VM_TYPE_ENTRY,
                  GENERATE_C1_TOPLEVEL_VM_TYPE_ENTRY,
                  GENERATE_C2_VM_TYPE_ENTRY,
                  GENERATE_C2_TOPLEVEL_VM_TYPE_ENTRY)

  GENERATE_VM_TYPE_LAST_ENTRY()
};

程式碼來自jdk8

Answer 1

static VMStructEntry localHotSpotVMStructs[];
  |         |               |              |
  |         |               |              |
  |         |               |              |
 静态      类型            变量名           数组 
 

去看看 VMStructEntry 的定義。不論如何，這就是一個數組，是一個 VMStructEntry 類型的數組。