[llvm-dev] Is it time to allow StringRef to be constructed from nullptr?

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Sun Sep 25 09:10:24 PDT 2016

While porting LLDB over to StringRef, I am continuously running into
difficulties caused by the fact that StringRef cannot be constructed from
nullptr.  So I wanted to see peoples' thoughts on removing this restriction
from StringRef.  To be clear, I'm only using LLDB as a motivating example,
but I'm not requesting that it be done because LLDB is some kind of special
case.  If it is to be done it should be on its own merits.  That said, here
is some context:

LLDB has a lot of functions that look like this:

void foo(const char *, Bar, const char *).

I'm trying to port these to functions that look like this:

void foo(StringRef, Bar, StringRef).

Often times the parameters are string literals or char arrays, but equally
often they are another const char* that got passed into the calling
function, or a return value from a CRT function like strstr(), or many
other possible sources.  This latter category presents a problem for
porting code to StringRef, because if I simply change the function
signature and fix up compile errors, I will probably have introduced a bug
because hundreds of callers will now be implicitly converting from const
char* to StringRef, leaving open the possibility that one of those was null.

To work around this, I've started doing the following every time I port a

void foo(const char *, Bar, const char*) = delete;

This is pretty hackish, but it gets the job done.  At least the compiler
warns me and forces me to go inspect every callsite where there's an
implicit conversion.  Unfortunately it also makes for extremely verbose
code.  Now instead of:

foo("bar", baz, "buzz")

I have to write

foo(StringRef("bar"), baz, StringRef("buzz"))

even for string literals and char arrays, which will obviously never be
null!  If StringRef would handle a null argument gracefully, it would make
my life much easier.

With that out of the way, here are some reasons I can see to allow
StringRef accept null to its constructor which are independent of LLDB and
stand on their own merit.

1) std::string_view<> can be constructed with null.  I don't know when we
will be able to use std::string_view<>, but there's a chance that at some
point in the future we may wish to remove StringRef in favor of
string_view.  That day isn't soon, but in any case, it will be easier if
our assumptions are the same.

2) [nullptr, nullptr+0) is a valid range.  Why shouldn't we be able to
construct a StringRef from an otherwise perfectly valid range?

3) StringRef() can *already* be constructed from nullptr (!)  Surprised?
That's what happens when you invoke the default constructor.  It happily
initializes the internal Data with null.  So why not allow the same
behavior when invoking the const char * constructor?

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