c-宏不通过直接调用扩展,而是通过间接扩展

我有以下宏

#include <boost/preprocessor.hpp>

#define DB_FIELD(...) BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)

#define DB_TOFIELD(type,name) \
  private:\
  type name##_;\
  public:\
  const type& get_##name(){return name##_;}\
  void set_##name(const type& val) { name##_ = val; }

#define GEN_ENUM_FIELD(r,data,elem) BOOST_PP_CAT(FIELD_,BOOST_PP_SEQ_ELEM(1,elem)),

#define DECLARE(type, name) DB_TOFIELD(type, name)

#define GEN_FIELD_DECL(r, data, elem) DECLARE(BOOST_PP_SEQ_ELEM(0,elem),BOOST_PP_SEQ_ELEM(1,elem))

#define DB_TABLE(name, ...) class name : public DataBaseTable {\
  public:\
  constexpr static const char *get_table_name() { return #name; }\
  BOOST_PP_LIST_FOR_EACH(GEN_FIELD_DECL,, BOOST_PP_VARIADIC_TO_LIST(__VA_ARGS__)) \
  enum Fields{ \
  BOOST_PP_LIST_FOR_EACH(GEN_ENUM_FIELD,, BOOST_PP_VARIADIC_TO_LIST(__VA_ARGS__))\
  FIELD_COUNT\
  };\
  };

现在下面的代码：

DB_TABLE(Test2,
  DB_FIELD(int, foo),
  DB_FIELD(int, bar)
)

产生：

class Test2 : public DataBaseTable {
public:
  constexpr static const char *get_table_name() { return "Test2"; }
private:
  int foo_;
public:
  const int &get_foo() { return foo_; }
  void set_foo(const int &val) { foo_ = val; }
private:
  int bar_;
public:
  const int &get_bar() { return bar_; }
  void set_bar(const int &val) { bar_ = val; }
  enum Fields { FIELD_foo, FIELD_bar, FIELD_COUNT };
};

正如我写的那样丑陋,但是我担心的是为什么在GEN_FIELD_DECL宏中需要这种级别的间接(DECLARE)？直接调用DB_TOFIELD

#define GEN_FIELD_DECL(r, data, elem) DB_TOFIELD(BOOST_PP_SEQ_ELEM(0,elem),BOOST_PP_SEQ_ELEM(1,elem))

产生垃圾：

class Test2 : public DataBaseTable {
public:
  constexpr static const char *get_table_name() { return "Test2"; }
private:
  int foo _;
public:
  const int &get_BOOST_PP_SEQ_ELEM(1, (int)(foo))() { return foo _; }
  void set_BOOST_PP_SEQ_ELEM(1, (int)(foo))(const int &val) { foo _ = val; }
private:
  int bar _;
public:
  const int &get_BOOST_PP_SEQ_ELEM(1, (int)(bar))() { return bar _; }
  void set_BOOST_PP_SEQ_ELEM(1, (int)(bar))(const int &val) { bar _ = val; }
  enum Fields { FIELD_foo, FIELD_bar, FIELD_COUNT };
};

clang 3.7.1和gcc 5.3再现了相同的行为

解决方法:

如果它们是#或##运算符的参数,则您遇到的是预处理器如何扩展参数的例外.从C .2011§16.3.1¶1：

After the arguments for the invocation of a function-like macro have been identified, argument substitution
takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or
followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros
contained therein have been expanded. Before being substituted, each argument’s preprocessing tokens are
completely macro replaced as if they formed the rest of the preprocessing file; no other preprocessing tokens
are available.

宏间接调用避免了exception子句,从而导致参数在被另一个宏处理之前被扩展.

例如：

#define FOO 10
#define BAR(x) x ## 7
#define BAR2(x) BAR(x)

int x = BAR(FOO);      // => int x = FOO7;

int y = BAR2(FOO);     // => int y = BAR(10); (intermediate result)
                       // => int y = 107;     (final expansion)

标签：boost-preprocessor,variadic-macros,c,macros
来源： https://codeday.me/bug/20191013/1909327.html