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

[Hal Finkel]

Sam,

Two quick questions:

1. Do you check the return type of c_str() to make sure it matches what
you expect? If you do check the type and it does not match, you should
also check if the returned type has a conversion operator to the type
needed.

2. Does this handle wide-character strings?

I like the idea of adding this note, I think it will be helpful for
many users.

 -Hal

On Fri, 1 Jun 2012 14:59:42 -0700
Sam Panzer <panzer at google.com> wrote:

> Hi,
> 
> We had a request for more user-friendly printf() diagnostics, which
> asked for a note suggesting a call to .c_str() when passing a c++
> string to printf when a char* is expected, and I thought it would
> make a good first submission:
> 
> Currently code such as
> >
> >  string foo;
> >
> >  printf("Oh dear %s", foo);
> >
> > produces a reasonable (if somewhat technical) diagnostic:
> >
> >  error: cannot pass object of non-trivial type 'string' (aka
> >> 'basic_string<char>') through variadic function; call will abort
> >> at runtime [-Wnon-pod-varargs]
> >
> >    printf("Oh dear %s", foo);
> >
> >
> >> That confuses some people (such as people who happened to be
> >> sitting near me today).  It would be easy for them to understand
> >> if this could special-case the situation where the call is to a
> >> *printf-like function and the non-POD in question is a string (or
> >> std::string), to suggest that the solution might be to add the
> >> missing ".c_str()".
> >
> >
> >> When format string checking is active for the function/argument in
> >> question it would also be possible to report on the expected
> >> argument type.
> >
> >
> >> (Of course it's also not ideal to report that the "call will abort
> >> at runtime" when generating an error rather than a warning, but
> >> that might not be worth fixing.)
> >
> >
> I attached a patch which emits this note when an object defining
> a .c_str() member function is passed instead of a char*, along with a
> test file. Searching for c_str() was intended to allow the note to
> fire for std::string replacements.
> 
> -Sam



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory