Boost.Preprocessor index list with macro

Question

I am trying to use Boost.Preprocessor to do some compile-time work. I want to index a table using values that are computed in other macros. When I try I get the following error: "concatenation with '(' in macro 'BOOST_PP_BOOL_I' does not create a valid token."

This is the simplest code that produces the issue.

#define MY_TABLE (0, (1, BOOST_PP_NIL))
#define MY_INDEX_FUNCTION(x) (x)
void func() {
    int y = BOOST_PP_LIST_AT(MY_TABLE, MY_INDEX_FUNCTION(0));
}

It is pretty easy to determine that removing the parens in MY_INDEX_FUNCTION resolves the issue in this case. My actual code uses a much more complex function to calculate the table index in a much larger table.

Is there something that I can do or change that would fix this such that the parens and more complex macros don't cause problems?

Answer 1

The second parameter of BOOST_PP_LIST_AT takes an index/integer. It works with tricky preprocessor hacks under the hood. The parameter(expanded) should be exactly an integer-literal, not an integer inside parenthesis. The MY_INDEX_FUNCTION should be changed, so that the parameter passed to the BOOST_PP_LIST_AT is literally an integer-literal:

#define MY_INDEX_FUNCTION(x)  x

The macro does not work with arithmetic expressions, this will not work:

#define MY_INDEX_FUNCTION(x) (x+1)
NOR
#define MY_INDEX_FUNCTION(x)  x+1

But you can do this with

#define MY_INDEX_FUNCTION(x) MY_INDEX_FUNCTION_ ## x
#define MY_INDEX_FUNCTION_0 1
#define MY_INDEX_FUNCTION_1 2
#define MY_INDEX_FUNCTION_2 3
//...

This macro definitions can be created by a (python-)script

def my_index_function(x):
    # insert the behavior of the macro here
    return x+1

MACRO_NAME = "MY_INDEX_FUNCTION"
INDEX_MAX = 255

for x in range(INDEX_MAX):
    print("#define %s_%i %i" % (
        MACRO_NAME,
        x,
        my_index_function(x),
    ))

print("#define %s(x) %s_ ## x" % (
    MACRO_NAME,
    MACRO_NAME,
))