Re: [Qemu-devel] [PATCH v2 1/8] CODING_STYLE: Section about conditional statement

[Eric Blake]

On 07/31/2014 08:32 PM, address@hidden wrote:
> From: Gonglei <address@hidden>
> 
> Yoda conidtions lack of readability, and QEMU have a

s/conidtions/conditions/
s/of //
s/have/has/

> strict compiler configuration for checking a common
> mistake like "if (dev = NULL)". Make it a written rule.
> 
> Signed-off-by: Gonglei <address@hidden>
> ---
>  CODING_STYLE | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/CODING_STYLE b/CODING_STYLE
> index 4280945..11a79d4 100644
> --- a/CODING_STYLE
> +++ b/CODING_STYLE
> @@ -91,3 +91,22 @@ Mixed declarations (interleaving statements and 
> declarations within blocks)
>  are not allowed; declarations should be at the beginning of blocks.  In other
>  words, the code should not generate warnings if using GCC's
>  -Wdeclaration-after-statement option.
> +
> +6. Conditional statement

s/statement/statements/

> +
> +Please don't use Yoda conditions because of lack of readability. Furthermore,
> +it is not the QEMU idiomatic coding style. Example:
> +
> +Usually a conditional statement in QEMU would be written as:
> +if (a == 0) {
> +    /* Reads like: "If a is equal to 0..." */
> +    do_something();
> +}
> +
> +Yoda conditions describe the same expression, but reversed:
> +if (0 == a) {
> +    /* Reads like: "If 0 equals to a" */
> +    do_something();
> +}
> +
> +The constant is listed first, then the variable being compared to.
> 

This spends more lines documenting the bad style than the good, and
doesn't quite flow with the rest of the document.  At the risk of
sounding like a complete rewrite, how about:

=====
When comparing a variable for (in)equality with a constant, list the
constant on the right, as in:

if (a == 0) {
    do_something();
}

Rationale: Yoda conditionals (as in 'if (0 == a)') are awkward to read.
Besides, good compilers already warn users when == is mis-typed as =,
even when the constant is on the right.
=====

and maybe some other ideas also worth adding:

=====
Avoid redundant comparisons: (bool_expr == true) is better written as
(bool_expr), and (ptr == NULL) is shorter as (!ptr).  Use of !!value is
a convenient shorthand for converting a value into a boolean.
=====

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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