[cfe-commits] [patch] Added a suggestion when a std::string is passed to printf()

Sam Panzer panzer at google.com
Mon Jun 4 18:17:12 PDT 2012

It turns out that intercepting the non-POD error is quite a bit more
complicated than getting the basic functionality of this patch to work, so
I decided to work on that as a separate patch.

Here is the newer patch. The important features include:
 - When a non-POD argument is passed to a printf-like function, it is
checked for a c_str() member function that takes no parameters and returns
a type that makes printf happy, the extra note is emitted with a FixItHint.
 - This works for standard and wide-char strings (and anything else that
people have thought up), since it lets analyze_printf::ArgTypeResult figure
out the type matching.
 - I don't check for conversion operators, since we (usually) get a char*
out of c_str().
 - Sema gained a function to find all C++ members of a given kind, a
generalization of what I used previously.

What I'm missing for Chandler's request of combining the error and warning
 - Code that detects that it is looking at a printf-like varargs function.
Something similar appears in lib/Sema/SemaChecking.cpp: it looks like
iterating over specific_attr_iterator<FormatAttr> as happens
in Sema::CheckFunctionCall is the best option. Is this correct?

Thanks again!

On Mon, Jun 4, 2012 at 2:10 PM, Chandler Carruth <chandlerc at google.com>wrote:

> On Mon, Jun 4, 2012 at 2:09 PM, Sam Panzer <panzer at google.com> wrote:
>> Now I'm wondering how much of this behavior should be "string"-specific.
>> If we're paying attention to conversion operators from non-POD types, why
>> not check for all possible conversion operators? I think this is a better
>> solution than checking to see if there's a conversion operator from the
>> return type of a c_str() function.
> I think conversion operators aren't really as interesting and at least
> should be handled as a separate follow-on patch. I also think they're
> completely uninteresting when considering the result of 'c_str()'. All of
> the interesting cases actually return the character pointer we're looking
> for.
>> Not depending on std::string was the plan, and in fact how I implemented
>> it :)
>> On Mon, Jun 4, 2012 at 1:44 PM, Chandler Carruth <chandlerc at google.com>wrote:
>>> On Mon, Jun 4, 2012 at 1:37 PM, Sam Panzer <panzer at google.com> wrote:
>>>> Here is one other question: With this patch, incorrectly passing an
>>>> std::string to printf generates an error (can't pass non-POD member), a
>>>> warning (printf format doesn't match), and a note (the c_str() suggestion).
>>>> Is this the behavior we want?
>>> I don't think so... Ideally:
>>> 1) If we are about to issue a an error (non-POD object passed through
>>> printf), check whether the non-POD object has a 'c_str()' method that
>>> returns a type which matches the format specifier. If so, use a specific
>>> diagnostic message for the error, attach a fixit-hint suggesting
>>> '.c_str()', and continue parsing as-if the user had done that.
>>> 2) If there is no error (passing a std::string* perhaps), then in the
>>> warning message, give a more precise message than 'cannot convert
>>> std::string* to const char*' or whatever by checking if there is a c_str()
>>> method that matches the type of the printf.
>>> One thing I would encourage you to do: don't base this on 'std::string'.
>>> I actually think the error/warning should fire for any class type with a
>>> c_str method that returns a viable type. That way non-standard string
>>> libraries will get the same benefit if they conform the the same conceptual
>>> interface as the standard string library.
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