Redundant macro usage in initializer lists

问题

After I were able to successfully reduce code duplication with this I want to further reduce my code duplication with totally different problem

#include <iostream>
#include <array>

#define A1
#define B2
#define C3
#define D4
#define E5


enum Enum {
#ifdef A1
  one,
#endif
#ifdef B2
  two,
#endif
#ifdef C3
  three,
#endif
#ifdef D4
  four,
#endif
#ifdef E5
  five,
#endif
  end
};

const char* map(Enum e)
{
  switch(e)
  {
    #ifdef A1
    case one  : return "one";
    #endif
    #ifdef B2
    case two: return "two";
    #endif
    #ifdef C3
    case three : return "three";
    #endif
    #ifdef D4
    case four: return "four";
    #endif
    #ifdef E5
    case five: return "five";
    #endif
    case end: return "end";
  }
}

int main()
{
  std::array<const char*, Enum::end + 1> arr{{
    #ifdef A1
      "a",
    #endif
    #ifdef B2
      "b",
    #endif
    #ifdef C3
      "c",
    #endif
    #ifdef D4
      "d",
    #endif
    #ifdef E5
      "e",
    #endif
      "z"
  }};
  int e = 0;
  for (const auto& str : arr)
  {
    std::cout << str << " map " << map(Enum(e)) << std::endl;
    e++;
  }
  return 0;
}

Please note the function map doesn't really exists in my code and was implemented only for this example. In the code above I have two array and their values initialized with initialization list. The problem I want to address is the need to have #ifdef and #endif twice. Instead, I want to have some thing like

#ifdef A1
#define A1_PAIR one, "a",
#else
#define A1_PAIR
#endif
#ifdef B2
#define B2_PAIR two, "b",
#else
#define B2_PAIR
#endif
#ifdef C3
#define C3_PAIR three, "c",
#else
#define C3_PAIR
#endif
#ifdef D4
#define D4_PAIR four, "d",
#else
#define D4_PAIR
#endif
#ifdef E5
#define E5_PAIR five, "e",
#else
#define E5_PAIR
#endif

#define MACRO(index) \
    A1_PAIR \
    B2_PAIR \
    C3_PAIR \
    D4_PAIR \
    E5_PAIR \
    end, "z"

enum Enum{ MACRO(1) };
std::array<const char*, Enum::end 1> arr2{{ MACRO(2) }};

Obviously, this code doesn't work but can I write something in this spirit?

回答1:

Create one array:

using arr_type=char const[];
#define ARRAY_SOURCE = arr_type{ "1", "a", "2", "b", /* etc */ }

use #ifdef to define indexes.

// elements.inc:
#ifdef A
ARRAY_SOURCE[0+OFFSET],
#endif
// etc

then

std::array<const char*, NUM + 1> arr1{{
#define OFFSET 0
#include "elements.inc"
#undef OFFSET
}};
std::array<const char*, NUM + 1> arr2{{
#define OFFSET 1
#include "elements.inc"
#undef OFFSET
}};

回答2:

You might use pair/tuple to group data together:

std::array<std::pair<const char*, const char*>, NUM + 1> arr{{
#ifdef A1
    {"1", "a"},
#endif
#ifdef B2
    {"2", "b"},
#endif
#ifdef C3
    {"3", "c"},
#endif
#ifdef D4
    {"4", "d"},
#endif
#ifdef E5
    {"5", "e"},
#endif
    {"26", "z"}
}};
for (const auto& p : arr) {
    std::cout << p.first << std::endl;
}
for (const auto& str : arr) {
    std::cout << p.second << std::endl;
}
std::cout << "ok" << std::endl;

来源：https://stackoverflow.com/questions/47752997/redundant-macro-usage-in-initializer-lists

标签

c++

c++11

c-preprocessor