How to convert source_set.add(when) into if statement? Doc is unclear

[hehaoqian]

I am beginner to meson build,
and just begin my QEMU development, which uses meson

In QEMU, there are many code similar to this for conditional compilation

ss = import('sourceset')
ss.add(when: 'CONFIG_XXXX', if_true: files('a.c'))
# The above code works as intended

But how can I check if 'CONFIG_XXXX' is true in an if statement,
so I can put multiple meson expression guarded by a config

What I tried:

if CONFIG_XXXX
endif

if 'CONFIG_XXXX'
endif

if config['CONFIG_XXXX']
endif

if configuration_data().get['CONFIG_XXXX']
endif

None of the above works, error during meson configure

The doc of source_set says

source_set.add([when: varnames_and_deps],
[if_true: sources_and_deps],
[if_false: list_of_alt_sources])
varnames_and_deps is a list of conditions for the rule, which can be either strings or dependency objects
(a dependency object is anything that has a found() method).
If all the strings evaluate to true and all dependencies are found,
the rule will evaluate to true;

I am very confused that how source_set.add(when) resolves string into boolean value?
A raw string in meson is not directly convertible to boolean in meson,
but how is it converted to bool in source_set.add(when)?

I guess the string is a key to a dictionary? But how to get the dictionary?
The documentation is unclear here

The exact QEMU commit I use is XUANTIE-RV/qemu@fd25f40
It uses meson 0.55.3,
and the meson commit is 776acd2

[dcbaker]

Those values are resolved when source_set.apply() is called, but are stored as strings until that time.

The basic opration is something like this:

conf = configuration_data()
conf.set('FOO_TRUE', true)
conf.set('FOO_FALSE', false)

s = source_set()
s.add(when : 'FOO_TRUE', files: files('true.c'))
s.add(when : 'FOO_FALSE', files : files('false.c'))

sources = s.apply(conf).sources()
assert(sources == files('true.c')

So if you want to change that into a conditional inline, you need to have whichever configuration_data that will be passed to apply:

conf = configuration_data()
conf.set('FOO_TRUE', true)
conf.set('FOO_FALSE', false)

if conf.get('FOO_TRUE')
  ...
endif