setq in init file not overriding the default for indent-tabs-mode

Question

It works when using setq-default in place of setq or when customizing through C-h v. C-h v reports that it is set in 'C source code'.

I was thinking it might be affected by my c-default-style, but my setq appears after it:

(setq c-default-style                   
              '((c-mode . "stroustrup") 
                (other . "stroustrup")))

...

(setq indent-tabs-mode nil)

I have made sure I am not using smart tabs or any other tab settings.

My understanding of setq:
1. Creates and sets the local variable if not present.
2. Sets local variable if present.
3. Does not affect default.

My understanding of setq-default:
1. If local variable is present only the default is affected.
2. If no local variable present default and local are set.

With this understanding in mind and assuming the local variable takes precedence, why would changing the default value have an effect, but not changing the local value?

Answer 1

You ask, "assuming the local variable takes precedence, why would changing the default value have an effect, but not changing the local value?"

If you use setq in your init file, and if the variable is buffer-local in the buffer that is current when your init file is evaluated, then only the value in that buffer gets set.

Probably the problem is that the buffer you expect to be current is not current when that expression is evaluated.

C-h v indent-tabs-mode tells you this:

Automatically becomes buffer-local when set.

That means that it is buffer-local, always. And that means that if you want to set the default value for all buffers then you need to use setq-default. After setting that default value, you can use setq in any given buffer to set its value to something else.