c++ - C++ 中编译时生成的 block

Question

我的服务器中有一个结构，我想用 C++ 中的宏或模板生成它，因为它有很多冗余的东西:

struct MyBlock {
  void Merge(const MyBlock& from) {
    if (apple.HasData()) {
      apple.Merge(from.apple);
    }
    if (banana.HasData()) {
      banana.Merge(from.banana());
    }
    ...
  }

  void Clear() {
    apple.Clear();
    banana.Clear();
    ...
  }

  void Update(const SimpleBlock& simple_block) {
    if (simple_block.apple.Updated()) {
      apple.Add(simple_block.apple);
    }
    if (simple_block.banana.Updated()) {
      banana.Add(simple_block.banana);
    }
    ...
  }
  Fruit apple;
  Fruit banana;
  Animal dog;
  Animal cat;
  ...
}

struct SimpleBlock {
  SimpleFruit apple;
  SimpleFruit banana;
  SimpleAnimal dog;
  SimpleAnimal cat;
  ...;
}

我想在 apple 和 dog 这两个 block 中定义更多变量。我还想定义更多对这样的 block 。但是涉及到很多琐碎的工作。所以我的问题是我们如何使用宏、模板或其他一些 C++ 功能(包括 C++11)在编译时生成这些 block ？

我不使用集合来存储这些变量的原因是因为 MyBlock 结构将作为参数传递到另一个模板类中，该模板类将在运行时动态分配和释放该 block 。它实际上是一个会定期聚合的线程本地 block 。

Answer 1

预处理器列表迭代足够简单:

#define M_NARGS(...) M_NARGS_(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define M_NARGS_(_10, _9, _8, _7, _6, _5, _4, _3, _2, _1, N, ...) N

#define M_CONC(A, B) M_CONC_(A, B)
#define M_CONC_(A, B) A##B
#define M_ID(...) __VA_ARGS__

#define M_LEFT(L, R) L
#define M_RIGHT(L, R) R

#define M_FOR_EACH(ACTN, ...) M_CONC(M_FOR_EACH_, M_NARGS(__VA_ARGS__)) (ACTN, __VA_ARGS__)

#define M_FOR_EACH_0(ACTN, E) E
#define M_FOR_EACH_1(ACTN, E) ACTN(E)
#define M_FOR_EACH_2(ACTN, E, ...) ACTN(E) M_FOR_EACH_1(ACTN, __VA_ARGS__)
#define M_FOR_EACH_3(ACTN, E, ...) ACTN(E) M_FOR_EACH_2(ACTN, __VA_ARGS__)
#define M_FOR_EACH_4(ACTN, E, ...) ACTN(E) M_FOR_EACH_3(ACTN, __VA_ARGS__)
#define M_FOR_EACH_5(ACTN, E, ...) ACTN(E) M_FOR_EACH_4(ACTN, __VA_ARGS__)
//.. extend this to higher numbers with some copy&paste


#define MYBLOCK(...) struct MyBlock { \
  void Merge(const MyBlock& from) { \
      M_FOR_EACH(BLOCK_MERGE, __VA_ARGS__) \
  } \
  void Clear() { \
      M_FOR_EACH(BLOCK_CLEAR, __VA_ARGS__) \
  } \
  void Update(const SimpleBlock& simple_block) { \
      M_FOR_EACH(BLOCK_UPDATE, __VA_ARGS__) \
  } \
  M_FOR_EACH(BLOCK_FIELD, __VA_ARGS__) \
}

#define BLOCK_MERGE(F) if (M_ID(M_RIGHT F).HasData()) { \
  M_ID(M_RIGHT F).Merge(from.M_ID(M_RIGHT F)); \
}
#define BLOCK_CLEAR(F) M_ID(M_RIGHT F).Clear;
#define BLOCK_UPDATE(F) if (simple_block.M_ID(M_RIGHT F).Updated()) { \
  M_ID(M_RIGHT F).Add(simple_block.M_ID(M_RIGHT F)); \
}
#define BLOCK_FIELD(F) M_ID(M_LEFT F) M_ID(M_RIGHT F);


#define SIMPLEBLOCK(...) struct SimpleBlock { M_FOR_EACH(SIMPLE_DECL, __VA_ARGS__) }
#define SIMPLE_DECL(F) M_CONC(Simple, M_ID(M_LEFT F)) M_ID(M_RIGHT F);

#define FIELDS (Fruit, apple),(Fruit,banana),(Animal,dog),(Animal,cat)

MYBLOCK(FIELDS);
SIMPLEBLOCK(FIELDS);

以现有格式向 FIELDS 添加必要的其他成员变量，它们将被添加到 MYBLOCK 和 SIMPLEBLOCK 发出的结构中. (记得用更多的迭代来扩展 M_FOR_EACH...用几个 ctrl+c，ctrl+v 很容易做到。)