I was trying to create and initialize a struct using a variable length macro. I want to be able create/initialize the struct with '0's and then initialize some fields of the struct with values that were optionally passed in.
Is this possible to do? I was trying the following:
#define INIT_ENTRY(x, ...) \
entry_t x = {.id = 0, .age = 0} \
x.id = <I want to use arg1 here>
x.age = <i want to use arg2 here if it exists>
Is there a way to accomplish this?
It can be done with a combination of argument counting and concatenation.
First you need a macro that counts it's arguments, which I'll call NARGS This was asked and answered here. Note that answers are compiler-dependent.
Next you need a series of macros for all specific cases:
#define INIT_ENTRY_0(x) entry_t x = {.id = 0, .age = 0}
#define INIT_ENTRY_1(x, _id) entry_t x = {.id = _id, .age = 0}
#define INIT_ENTRY_2(x, _id, _age) entry_t x = {.id = _id, .age = _age}
You also need a concatenation macro:
#define CONCAT(a, b) a##b
Last, use concatenation to choose the correct one:
#define INIT_ENTRY(x, ...) CONCAT(INIT_ENTRY_, NARGS(__VA_ARGS__))(x, #__VA_ARGS__)
If you keep the same order of initializers in your macro as in your initialization list, then given you're always initializing everything else to 0, you can simply do this:
#define _INIT_ENTRY_UTILITY(x, ID_, AGE_, ...) \
entry_t x = {.id = ID_, .age = AGE_}
#define INIT_ENTRY(...) _INIT_ENTRY_UTILITY(__VA_ARGS__, 0, 0, )
I assume your real use case has more than 2 parameters; if you take this approach, make sure to mimic its form exactly to maintain portability. In particular:
#define INIT_ENTRY(x, ...) for "clarity" will break calls that look like INIT_ENTRY(foo) on standard C preprocessors. What's worse is that it will actually work on some popular cpp's (such as gnu) that support such things as an extension, so the portability issue's hidden._INIT_ENTRY_UTILITY from INIT_ENTRY's replacement list leads to the same portability issue
x = INIT_ENTRY(x, 1)would do about(entry_t){.id = 1}andx = INIT_ENTRY(x, 1, 2)would do about(entry_t){.id = 1, .age = 2}? What is the max length needed - arbitrary? BTW:x.id = <I want to use arg1 here>is not initialization, but assignment.