Setting macro-variable-value in macro-function C++

Question

I need to call a function which call a macro-function to change macro-value in runtime.

This code isn't compiled:

#define MY_MACRO 32
#define SET_MY_MACRO_VAL(IS_TRUE)(MY_MACRO=(IS_TRUE)?16:U32)

In function SET_MY_MACRO_VAL

> error: lvalue required as left operand of assignment

    #define SET_MY_MACRO_VAL(IS_TRUE)(MY_MACRO=(IS_TRUE)?16:U32)
                                          ^
    in expansion of macro 'SET_MY_MACRO_VAL'
         SET_MY_MACRO_VAL(True);
         ^

there are no macros at runtime. Macros are expanded even before compilation — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jun 5, 2018 at 9:00
Is IS_TRUE known at compile time? If not I'm afraid using the preprocessor is a non-starter — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jun 5, 2018 at 9:05
There are no such things as macro-functions or macro-variables. — n. m. could be an AI
– n. m. could be an AI, Commented Jun 5, 2018 at 9:10

Clonk · Accepted Answer · 2018-06-05 09:16:35Z

3

Macro value are replaced BEFORE compile time by the preprocessor and do not exist at run time.

It is not a variable it is simply a way of using text for the value "32".

If you do this :

#define MY_MACRO 32
#define SET_MY_MACRO_VAL(IS_TRUE)(MY_MACRO=(IS_TRUE)?16:U32)

It will be expanded to this

#define MY_MACRO 32
#define SET_MY_MACRO_VAL(IS_TRUE)(32=(IS_TRUE)?16:U32)

What you can do is use a #define

#ifdef SET_MACRO_VAL_32
#define MY_MACRO 32
#else
#define MY_MACRO 16
#endif

Or use a conditionnal macro if you prefer

#if (IS_TRUE>0)
#define MY_MACRO 32
#else
#define MY_MACRO 16
#endif

Edit : In C++, you shouldn't really need macro though. You can use template and / or constexpr variable for compile-time value. In C++17 you can even use constexpr if.

edited Jun 5, 2018 at 9:16

answered Jun 5, 2018 at 9:04

Clonk

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3 Comments

463035818_is_not_an_ai Over a year ago

..or not use macros at all

Clonk Over a year ago

Yes but the question was about macro :). In C++ template and/or constexpr are way better and cleaner

463035818_is_not_an_ai Over a year ago

one could also read the question as "how to call function to change a value at runtime" ;) dont worry, you got my upvote already

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