Check for cache

[axx]

Hi,

Newbie question. I'm trying to understand the code in http_server.hpp and I'm having a hard time understanding line 172:

bool b = true;
((b&&(b = need_cache(std::forward<AP>(ap))), false),...);

Could you please explain why there's a false? Eclipse tells me there's a syntax error, by the way.

Thanks in advance,
Allister

[qicosmos]

It's fold expression in c++17, you need choose c++17.

[axx]

OK, I found this documentation: https://en.cppreference.com/w/cpp/language/fold.
So just to confirm that I understood correctly, line 172 corresponds to a unary right fold:

( pack op ... )

where op is the comma operator and pack is (b&&(b = need_cache(std::forward<AP>(ap))), false)?

[qicosmos]

the op is the comma expression

[axx]

While I still can't wrap my head around this fold expression, I tried to see what it does, so I wrote this code:
https://gist.github.com/axx/4160b06a0f3e2832c1b2c5087870db4a
The output is like this:

./a.out                  
do_something(log_t{})
does not have type enable_cache
==========
do_something(enable_cache{ false })
skip cache
==========
do_something(enable_cache{ true })
need cache
==========
do_something(log_t{}, enable_cache{ false })
skip cache
==========
do_something(log_t{}, enable_cache{ true })
skip cache
==========

The last function call looks weird to me, since I passed a enable_cache{ true } but we still eventually skip cache (because of the log_t{}).
Is that the intended behavior?

[qicosmos]

I will explain the intention for you tomorrow:)

[qicosmos]

Well, there is bug here, i have fixed it right now, thanks for your checking.
Ok, i would like to explain the intention and code now.

server.set_http_handler<GET, POST>("/a", &person::foo, enable_cache{ false }, log_t{});
server.set_http_handler<GET, POST>("/b", &person::foo1, enable_cache{ true }, log_t{});
server.set_http_handler<GET, POST>("/b", &person::foo1, log_t{}, enable_cache{ true });

Firstly, i don't want to limit the sequence of enable_cache, as you can see, the enbale_cache can be every where;
Secondly, i need to check the real value of enable_cache, maybe true, maybe false. I need to check it's true or false, furthermore, if the argument is not enable_cache, it is false.
Thirdly, i want to do an optimization, if the enable_cache value is true, i will finish checking the cache value, so i use a bool value to control the checking.

    server.set_http_handler<GET, POST>("/a", &person::foo, log_t{});
server.set_http_handler<GET, POST>("/a", &person::foo, enable_cache{ false }, log_t{});
server.set_http_handler<GET, POST>("/b", &person::foo1, enable_cache{ true }, log_t{});
server.set_http_handler<GET, POST>("/b", &person::foo1, log_t{}, enable_cache{ true });

In the first line, the need_cache will be false, as there is not enable_cache;
In the second line , the need_cache will be false, as the enable_cache value is false;
In the third line, the need_cache will be true, as the enable_cache value is true, and i won't do check for the next arguments, because i have got the true enable_cache.
In the last line, the need_cache will be true, as the enable_cache value is true; when i check the first argument log_t{}, the result is false, i will continue to check the remaining arguments until it' true.

[axx]

Thanks for the explanation. I think I finally understood the logic here. That said, I think this line could still be simplified from:

((!b&&(b = need_cache(std::forward<AP>(ap))), false),...);

to this (removing the false expression since it's not doing anything):

((!b&&(b = need_cache(std::forward<AP>(ap)))),...);

[qicosmos]

yes, you're right, i will remove the false expression.

[axx]

Cool! I'm closing this issue then. Thanks for your patience :)