C programming, make change at two location with one macro - c

Question

struct Error
{
MACRO(1, Connect);
MACRO(2, Timeout);
};
I need to define MACRO() in such way that the above code will generate the following code.
struct Error
{
static const int Connect = 1;
static const int Timeout = 2;
const char * const name[] = {"Connect", "Timeout"};
};
Is this possible or what is the alternative to get what I'm trying to do?

Answer 1

You can't do this directly, but you can if you move the macros to a separate location (such as a separate file):
macros.hpp
MACRO(1, Connect)
MACRO(2, Timeout)
#undef MACRO
the other file
struct Error
{
#define MACRO(a, b) static const int b = a;
#include "macros.hpp"
const char * const name [] = {
#define MACRO(a, b) #b,
#include "macros.hpp"
}
};
Alternatively, you could achieve a similar effect with Boost.Preprocessor.

Answer 2

Here's a Boost.Preprocessor solution:
#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/seq/size.hpp>
#include <boost/preprocessor/tuple/elem.hpp>
#include <boost/preprocessor/stringize.hpp>
#define FIRST(a, b) a
#define SECOND(a, b) b
#define DECLARE_VAR(r, data, elem) \
static const int FIRST elem = SECOND elem;
#define NAME_ARRAY_ELEM(r, data, elem) \
BOOST_PP_STRINGIZE(FIRST elem),
#define MACRO(seq) \
BOOST_PP_SEQ_FOR_EACH(DECLARE_VAR, ~, seq) \
const char * const name[] = { \
BOOST_PP_SEQ_FOR_EACH(NAME_ARRAY_ELEM, ~, seq) \
}
int main()
{
MACRO(((Connect, 1))((TimeOut, 2)));
return 0;
}
You have to make sure to double bracket each ((Token, value)) pair, however you don't need a separate file for your macro.

Answer 3

What you want, is to have a single list, that will automatically generate the definition and the name list, correct?
If so, search for X Macros in google.
Example:
#define EXPAND_AS_DEFINITION(a, b) static const int b = a;
#define EXPAND_AS_ARRAY(a, b) #b,
#define STATE_TABLE(ENTRY) \
ENTRY(1, Connect) \
ENTRY(2, Timeout)
struct Error
{
STATE_TABLE(EXPAND_AS_DEFINITION)
static const char * const name[];
};
const char * const Error::name[] = {STATE_TABLE(EXPAND_AS_ARRAY) 0};

Answer 4

It looks like like you are trying to define an enum Error that also has the strings as members. I will give you my own solution to this problem. (I'm not addressing the question but I believe that my answer is relevant for what I understand that OP is trying to do.)
And I just realized that OP is targeting C, not C++, so not sure if this can be done...
In MyEnum.hpp
#define MYENUM(X,...) \
struct X { \
enum Enum {__VA_ARGS__}; \
static const std::vector<std::string> names; \
static X::Enum which(const std::string& s) { \
return static_cast<X::Enum>(findEnum(s,names)); \
} \
static std::string str(X::Enum i) { \
return names[i];} \
}
Here findEnum() is just a linear search over the vector that returns the position index (additionally, in my implementation if it doesn't find it it throws an exception with all the possible correct inputs, I also do case insensitive comparison). Note that an ordered map instead of a vector would be more efficient (O(log(n)) instead of O(n)), but I didn't cared much because the size of those things is very small in my case.
Below the previous macro, declare your enum as
MYENUM(Error,Connect,Timeout); // I put the semicolon here not in the macro
And in MyEnum.cpp, add
#include <boost/assign/list_of.hpp>
const std::vector<std::string> Error::names = boost::assign::list_of
("Connect")("Timeout");
(I think that it should be possible to use initialization lists with a modern compiler). The important thing here is to make sure that the order is the same, otherwise it will not work.
Then, you can do stuff like this:
Error::Enum err1 = Error::Connect;
Error::Enum err2 = Error::which("Timeout");
std::cout << "Got " << Error::str(err1) << " error. Not good.\n";