需要元编程魔术来以无错误的方式定义位字段的母体

Question

目标是控制允许哪些类型的用户在UI级别执行哪些操作.这段代码已经存在了一段时间; 我只想稍微改进一下.我试图改进的文件可能应该是自动生成的,但是这个变化太大了,所以我寻求一个更简单的解决方案.

我们将调用的文件PermissionBits.h有一堆这样的:

// Here names are mangled; for example XYZ_OP_A is:
// permission to operation A in category/context XYZ
// SCU64 = static const unsigned __int64
// Some namespaces utilize all 64 bits
// The actual values (as long as they are proper bit fields) 
// do not matter - they are always used by name
namespace XYZPermissionBits
{
    SCU64 XYZ_OP_A = 1UI64 <<  0; //    1 = 0x0000000000000001
    SCU64 XYZ_OP_B = 1UI64 <<  1; //    2 = 0x0000000000000002
    SCU64 XYZ_OP_C = 1UI64 <<  2; //    4 = 0x0000000000000004
    SCU64 XYZ_OP_C = 1UI64 <<  3; //    8 = 0x0000000000000008
    SCU64 XYZ_OP_D = 1UI64 <<  4; //   16 = 0x0000000000000010
    SCU64 XYZ_OP_E = 1UI64 <<  5; //   32 = 0x0000000000000020
    SCU64 XYZ_OP_F = 1UI64 <<  6; //   64 = 0x0000000000000040
    SCU64 XYZ_OP_G = 1UI64 <<  7; //  128 = 0x0000000000000080
    SCU64 XYZ_OP_H = 1UI64 <<  8; //  256 = 0x0000000000000100
    SCU64 XYZ_OP_I = 1UI64 <<  9; //  512 = 0x0000000000000200
    SCU64 XYZ_OP_J = 1UI64 << 10; // 1024 = 0x0000000000000400
    SCU64 XYZ_OP_K = 1UI64 << 11; // 2048 = 0x0000000000000800
    SCU64 XYZ_OP_L = 1UI64 << 12; // 4096 = 0x0000000000001000 
}

即使在1UI64 << <numBits>;快捷方式的帮助下仍然存在问题,因为编码器会创建具有重复值的标记,进行拼写错误等.

理想情况下,我想要一个可以很好地格式化的宏,看起来像:

BITFIELDS_FOR_NAMESPACE(
    //*************** <<== I want to make the namespace name more vivid
    XYZPermissionBits,
    //*************** <<== somehow if that is possible. It is not a must-have.
    XYZ_OP_A, // Being able to add a comment here would be nice, but not critical
    XYZ_OP_B,
    XYZ_OP_C,
    XYZ_OP_D,
    XYZ_OP_E,
    XYZ_OP_F,
    XYZ_OP_G,
    XYZ_OP_H,
    XYZ_OP_I,
    XYZ_OP_J,
    XYZ_OP_K,
    XYZ_OP_L
)

我希望这个宏是灵活的,并防止我输入少于2个或超过65个参数 - 命名空间名称+ 64个标志.是可以做我想要的或接近它,还是应该使用生成的代码？您有什么其他建议吗？

Answer 1

使用Boost.PreProcessor测试的示例:

#include <boost/preprocessor/seq/for_each_i.hpp>
#include <boost/preprocessor/comparison/greater.hpp>
#include <boost/preprocessor/comparison/less.hpp>
#include <boost/preprocessor/debug/assert.hpp>
#include <boost/preprocessor/seq/size.hpp>

#define CHECK_SIZE(size) \
  BOOST_PP_ASSERT_MSG(BOOST_PP_GREATER(size, 1), "<  2 :(") \
  BOOST_PP_ASSERT_MSG(BOOST_PP_LESS(size, 65),   "> 64 :(") \

#define DO_MAKE_BITFIELDS(a, b, i, elem) \
  SCU64 elem = 1UI64 << i;

#define BITFIELDS_FOR_NAMESPACE(name, seq) \
  CHECK_SIZE(BOOST_PP_SEQ_SIZE(seq)) \
  namespace name { \
    BOOST_PP_SEQ_FOR_EACH_I(DO_MAKE_BITFIELDS, _, seq) \
  }

用法:

BITFIELDS_FOR_NAMESPACE(
    XYZPermissionBits,
    (XYZ_OP_A)
    (XYZ_OP_B)
    // ...
);