Iterating over a struct in C++

Question

I have a structure

typedef struct A
{
    int a;
    int b;
    char * c;
}aA;

I want to iterate over each an every member of the structure

Answer 1

I wrote a version without Boost or other third-party library, which has been tested using GCC 4.9(c++11), clang 5.0(c++11), VS 2008, VS 2019.

#include 
#include 

#define REFLECTION_WITH_FIELD_NAME 1

#define _PP_EVAL(...) __VA_ARGS__
#define _PP_EAT(...)
#define _PP_EMPTY
#define _PP_STR2(x) #x
#define _PP_STR(x) _PP_STR2(x)

#define _PP_MAP01(f, x) f(x)
#define _PP_MAP02(f, x, ...) f(x) _PP_EVAL(_PP_MAP01(f, __VA_ARGS__))
#define _PP_MAP03(f, x, ...) f(x) _PP_EVAL(_PP_MAP02(f, __VA_ARGS__))
#define _PP_MAP04(f, x, ...) f(x) _PP_EVAL(_PP_MAP03(f, __VA_ARGS__))
#define _PP_MAP05(f, x, ...) f(x) _PP_EVAL(_PP_MAP04(f, __VA_ARGS__))
#define _PP_MAP06(f, x, ...) f(x) _PP_EVAL(_PP_MAP05(f, __VA_ARGS__))
#define _PP_MAP07(f, x, ...) f(x) _PP_EVAL(_PP_MAP06(f, __VA_ARGS__))
#define _PP_MAP08(f, x, ...) f(x) _PP_EVAL(_PP_MAP07(f, __VA_ARGS__))
#define _PP_MAP09(f, x, ...) f(x) _PP_EVAL(_PP_MAP08(f, __VA_ARGS__))
#define _PP_MAP10(f, x, ...) f(x) _PP_EVAL(_PP_MAP09(f, __VA_ARGS__))
#define _PP_MAP11(f, x, ...) f(x) _PP_EVAL(_PP_MAP10(f, __VA_ARGS__))
#define _PP_MAP12(f, x, ...) f(x) _PP_EVAL(_PP_MAP11(f, __VA_ARGS__))
#define _PP_MAP13(f, x, ...) f(x) _PP_EVAL(_PP_MAP12(f, __VA_ARGS__))
#define _PP_MAP14(f, x, ...) f(x) _PP_EVAL(_PP_MAP13(f, __VA_ARGS__))
#define _PP_MAP15(f, x, ...) f(x) _PP_EVAL(_PP_MAP14(f, __VA_ARGS__))
#define _PP_MAP16(f, x, ...) f(x) _PP_EVAL(_PP_MAP15(f, __VA_ARGS__))
#define _PP_MAP17(f, x, ...) f(x) _PP_EVAL(_PP_MAP16(f, __VA_ARGS__))
#define _PP_MAP18(f, x, ...) f(x) _PP_EVAL(_PP_MAP17(f, __VA_ARGS__))
#define _PP_MAP19(f, x, ...) f(x) _PP_EVAL(_PP_MAP18(f, __VA_ARGS__))
#define _PP_MAP20(f, x, ...) f(x) _PP_EVAL(_PP_MAP19(f, __VA_ARGS__))
#define _PP_MAP21(f, x, ...) f(x) _PP_EVAL(_PP_MAP20(f, __VA_ARGS__))
#define _PP_MAP22(f, x, ...) f(x) _PP_EVAL(_PP_MAP21(f, __VA_ARGS__))
#define _PP_MAP23(f, x, ...) f(x) _PP_EVAL(_PP_MAP22(f, __VA_ARGS__))
#define _PP_MAP24(f, x, ...) f(x) _PP_EVAL(_PP_MAP23(f, __VA_ARGS__))
#define _PP_MAP25(f, x, ...) f(x) _PP_EVAL(_PP_MAP24(f, __VA_ARGS__))
#define _PP_MAP26(f, x, ...) f(x) _PP_EVAL(_PP_MAP25(f, __VA_ARGS__))
#define _PP_MAP27(f, x, ...) f(x) _PP_EVAL(_PP_MAP26(f, __VA_ARGS__))
#define _PP_MAP28(f, x, ...) f(x) _PP_EVAL(_PP_MAP27(f, __VA_ARGS__))
#define _PP_MAP29(f, x, ...) f(x) _PP_EVAL(_PP_MAP28(f, __VA_ARGS__))
#define _PP_MAP30(f, x, ...) f(x) _PP_EVAL(_PP_MAP29(f, __VA_ARGS__))
#define _PP_MAP31(f, x, ...) f(x) _PP_EVAL(_PP_MAP30(f, __VA_ARGS__))
#define _PP_MAP32(f, x, ...) f(x) _PP_EVAL(_PP_MAP31(f, __VA_ARGS__))
#define _PP_MAP33(f, x, ...) f(x) _PP_EVAL(_PP_MAP32(f, __VA_ARGS__))
#define _PP_MAP34(f, x, ...) f(x) _PP_EVAL(_PP_MAP33(f, __VA_ARGS__))
#define _PP_MAP35(f, x, ...) f(x) _PP_EVAL(_PP_MAP34(f, __VA_ARGS__))
#define _PP_MAP36(f, x, ...) f(x) _PP_EVAL(_PP_MAP35(f, __VA_ARGS__))
#define _PP_MAP37(f, x, ...) f(x) _PP_EVAL(_PP_MAP36(f, __VA_ARGS__))
#define _PP_MAP38(f, x, ...) f(x) _PP_EVAL(_PP_MAP37(f, __VA_ARGS__))
#define _PP_MAP39(f, x, ...) f(x) _PP_EVAL(_PP_MAP38(f, __VA_ARGS__))
#define _PP_MAP40(f, x, ...) f(x) _PP_EVAL(_PP_MAP39(f, __VA_ARGS__))
#define _PP_MAP41(f, x, ...) f(x) _PP_EVAL(_PP_MAP40(f, __VA_ARGS__))
#define _PP_MAP42(f, x, ...) f(x) _PP_EVAL(_PP_MAP41(f, __VA_ARGS__))
#define _PP_MAP43(f, x, ...) f(x) _PP_EVAL(_PP_MAP42(f, __VA_ARGS__))
#define _PP_MAP44(f, x, ...) f(x) _PP_EVAL(_PP_MAP43(f, __VA_ARGS__))
#define _PP_MAP45(f, x, ...) f(x) _PP_EVAL(_PP_MAP44(f, __VA_ARGS__))
#define _PP_MAP46(f, x, ...) f(x) _PP_EVAL(_PP_MAP45(f, __VA_ARGS__))
#define _PP_MAP47(f, x, ...) f(x) _PP_EVAL(_PP_MAP46(f, __VA_ARGS__))
#define _PP_MAP48(f, x, ...) f(x) _PP_EVAL(_PP_MAP47(f, __VA_ARGS__))
#define _PP_MAP49(f, x, ...) f(x) _PP_EVAL(_PP_MAP48(f, __VA_ARGS__))
#define _PP_MAP50(f, x, ...) f(x) _PP_EVAL(_PP_MAP49(f, __VA_ARGS__))
#define _PP_MAP51(f, x, ...) f(x) _PP_EVAL(_PP_MAP50(f, __VA_ARGS__))
#define _PP_MAP52(f, x, ...) f(x) _PP_EVAL(_PP_MAP51(f, __VA_ARGS__))
#define _PP_MAP53(f, x, ...) f(x) _PP_EVAL(_PP_MAP52(f, __VA_ARGS__))
#define _PP_MAP54(f, x, ...) f(x) _PP_EVAL(_PP_MAP53(f, __VA_ARGS__))
#define _PP_MAP55(f, x, ...) f(x) _PP_EVAL(_PP_MAP54(f, __VA_ARGS__))
#define _PP_MAP56(f, x, ...) f(x) _PP_EVAL(_PP_MAP55(f, __VA_ARGS__))
#define _PP_MAP57(f, x, ...) f(x) _PP_EVAL(_PP_MAP56(f, __VA_ARGS__))
#define _PP_MAP58(f, x, ...) f(x) _PP_EVAL(_PP_MAP57(f, __VA_ARGS__))
#define _PP_MAP59(f, x, ...) f(x) _PP_EVAL(_PP_MAP58(f, __VA_ARGS__))
#define _PP_MAP60(f, x, ...) f(x) _PP_EVAL(_PP_MAP59(f, __VA_ARGS__))
#define _PP_MAP61(f, x, ...) f(x) _PP_EVAL(_PP_MAP60(f, __VA_ARGS__))
#define _PP_MAP62(f, x, ...) f(x) _PP_EVAL(_PP_MAP61(f, __VA_ARGS__))
#define _PP_MAP63(f, x, ...) f(x) _PP_EVAL(_PP_MAP62(f, __VA_ARGS__))
#define _PP_MAP64(f, x, ...) f(x) _PP_EVAL(_PP_MAP63(f, __VA_ARGS__))

#define _PP_GET_NTH_ARG( \
  _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, \
  _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, \
  _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, \
  _31, _32, _33, _34, _35, _36, _37, _38, _39, _40, \
  _41, _42, _43, _44, _45, _46, _47, _48, _49, _50, \
  _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, \
  _61, _62, _63, _64, N, ...) N

#define _PP_MAP(f, ...) _PP_EVAL(_PP_EVAL(_PP_GET_NTH_ARG(__VA_ARGS__, \
  _PP_MAP64, _PP_MAP63, _PP_MAP62, _PP_MAP61, \
  _PP_MAP60, _PP_MAP59, _PP_MAP58, _PP_MAP57, _PP_MAP56, \
  _PP_MAP55, _PP_MAP54, _PP_MAP53, _PP_MAP52, _PP_MAP51, \
  _PP_MAP50, _PP_MAP49, _PP_MAP48, _PP_MAP47, _PP_MAP46, \
  _PP_MAP45, _PP_MAP44, _PP_MAP43, _PP_MAP42, _PP_MAP41, \
  _PP_MAP40, _PP_MAP39, _PP_MAP38, _PP_MAP37, _PP_MAP36, \
  _PP_MAP35, _PP_MAP34, _PP_MAP33, _PP_MAP32, _PP_MAP31, \
  _PP_MAP30, _PP_MAP29, _PP_MAP28, _PP_MAP27, _PP_MAP26, \
  _PP_MAP25, _PP_MAP24, _PP_MAP23, _PP_MAP22, _PP_MAP21, \
  _PP_MAP20, _PP_MAP19, _PP_MAP18, _PP_MAP17, _PP_MAP16, \
  _PP_MAP15, _PP_MAP14, _PP_MAP13, _PP_MAP12, _PP_MAP11, \
  _PP_MAP10, _PP_MAP09, _PP_MAP08, _PP_MAP07, _PP_MAP06, \
  _PP_MAP05, _PP_MAP04, _PP_MAP03, _PP_MAP02, _PP_MAP01 \
  ))(f, __VA_ARGS__))

#if REFLECTION_WITH_FIELD_NAME
#define _PP_REFLECTION_FIELD_NAME(x) _PP_STR(_PP_EVAL(x))
#else
#define _PP_REFLECTION_FIELD_NAME(x) ""
#endif

#define _PP_REFLECTION_ALL(x) _PP_EVAL x
#define _PP_REFLECTION_SECOND(x) _PP_EAT x
#define _PP_REFLECTION_FIELD(x) _PP_REFLECTION_ALL(x);
#define _PP_REFLECTION_METHOD2(x) v(this, _PP_REFLECTION_FIELD_NAME(x), x);
#define _PP_REFLECTION_METHOD(x) _PP_REFLECTION_METHOD2(_PP_REFLECTION_SECOND(x))

#define _PP_REFLECTION_VISTOR_METHOD(type, ...) \
  template  \
  void _reflect(Vistor& v) type { \
    _PP_MAP(_PP_REFLECTION_METHOD, __VA_ARGS__) \
  }

#define REFLECT(...) \
  _PP_MAP(_PP_REFLECTION_FIELD, __VA_ARGS__) \
  _PP_REFLECTION_VISTOR_METHOD(_PP_EMPTY, __VA_ARGS__) \
  _PP_REFLECTION_VISTOR_METHOD(const, __VA_ARGS__)


// Usage of REFLECT()

#define OBJECT_NAME_METHOD(obj) \
  static const char* object_name() { \
    return #obj; \
  }

struct Demo
{
  OBJECT_NAME_METHOD(Demo)

  REFLECT(
    (int) a,
    (int) b,
    (std::string) c
  )

  int d; // DO NOT REFLECT
};

struct Amplifier {
  template 
  void apply(Obj* obj) {
    obj->_reflect(*this);
  }

  template 
  void operator() (Obj* /*obj*/, const char* /*name*/, Field& field) {
    field *= 100;
  }

  template 
  void operator() (Obj* /*obj*/, const char* /*name*/, std::string& field) {
    field += "00";
  }
};

struct Printer {
  template 
  void print(const Obj& obj) {
    obj._reflect(*this);
  }

  template 
  void operator() (Obj* obj, const char* name, const Field& field) {
    std::cout << obj->object_name() << "."
      << name << ": " << field << std::endl;
  }
};

int main() {
  Demo a = {100, 200, "300", 400};

  Amplifier amplifier;
  amplifier.apply(&a);

  Printer printer;
  printer.print(a);
}

/*
Output:
Demo.a: 10000
Demo.b: 20000
Demo.c: 30000
*/

BTW, the following code is used for generating all _PP_MAP_? macros (written in JS, so you can run it in web browser).

(function() {
  const maxNumOfEle = 64;
  const mapNamePrefix = '_PP_MAP';
  let codeText = '';

  function formatNumWidth(num) {
    return ("0" + num).slice(-2);
  }

  function AddNewLine() {
    if (codeText.slice(-1) != ' ') {
      codeText += ' ';
    }

    codeText += '\\\n';
    codeText += ' '.repeat(2);
  }

  codeText += `#define ${mapNamePrefix}${formatNumWidth(1)}(f, x) f(x)\n`;
  for (let i = 2; i <= maxNumOfEle; ++i) {
    let funId = formatNumWidth(i);
    codeText += `#define ${mapNamePrefix}${funId}(f, x, ...) f(x)`;

    let nextFunId = formatNumWidth(i - 1);
    codeText += ' _PP_EVAL(';
    codeText += `${mapNamePrefix}${nextFunId}(f, __VA_ARGS__)`;
    codeText += ')';

    codeText += '\n';
  }

  codeText += '\n#define _PP_GET_NTH_ARG(';
  AddNewLine();
  for (let i = 1; i <= maxNumOfEle; ++i) {
    codeText += `_${i}, `;
    if ((i % 10) == 0) {
      AddNewLine();
    }
  }
  codeText += 'N, ...) N\n';

  codeText += `\n#define ${mapNamePrefix}(f, ...) `;
  codeText += '_PP_EVAL(_PP_EVAL(_PP_GET_NTH_ARG(__VA_ARGS__,';
  AddNewLine();
  for (let i = maxNumOfEle; i >= 1; --i) {
    let funId = formatNumWidth(i);
    codeText += `${mapNamePrefix}${funId}`;
    if (i != 1) {
      codeText += ', ';
    }

    if ((i % 5) == 1) {
      AddNewLine();
    }
  }
  codeText += '))(f, __VA_ARGS__))\n';

  console.log(codeText);
})();