llvm dev - Apr 2019 - How does Twine work?

I'm looking at the documentation on Twine at
http://llvm.org/docs/ProgrammersManual.html#llvm-adt-twine-h and it gives
example code:

void foo(const Twine &T);
...
StringRef X = ...
unsigned i = ...
foo(X + "." + Twine(i));

How exactly does that last line work? Since addition is left associative, I
would expect it to be parsed as (X + ".") ... so it's trying to
add a
StringRef and a const char* before Twine can come into the picture at all?
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I believe StringRef + const char* is overloaded at the bottom of Twine.h.
And without those overloads it might try to implicitly convert both
StringRef and const char* to Twine and then add them since StringRef +
const char * wouldn't otherwise be defined.

  inline Twine operator+(const Twine &LHS, const Twine &RHS) {
    return LHS.concat(RHS);
  }

  /// Additional overload to guarantee simplified codegen; this is
equivalent to
  /// concat().

  inline Twine operator+(const char *LHS, const StringRef &RHS) {
    return Twine(LHS, RHS);
  }

  /// Additional overload to guarantee simplified codegen; this is
equivalent to
  /// concat().

  inline Twine operator+(const StringRef &LHS, const char *RHS) {
    return Twine(LHS, RHS);
  }



~Craig


On Mon, Apr 29, 2019 at 3:18 PM Russell Wallace via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I'm looking at the documentation on Twine at
> http://llvm.org/docs/ProgrammersManual.html#llvm-adt-twine-h and it gives
> example code:
>
> void foo(const Twine &T);
> ...
> StringRef X = ...
> unsigned i = ...
> foo(X + "." + Twine(i));
>
> How exactly does that last line work? Since addition is left associative,
> I would expect it to be parsed as (X + ".") ... so it's
trying to add a
> StringRef and a const char* before Twine can come into the picture at all?
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Ah, that makes sense. Thanks!

On Mon, Apr 29, 2019 at 11:43 PM Craig Topper <craig.topper at gmail.com>
wrote:
> I believe StringRef + const char* is overloaded at the bottom of Twine.h.
> And without those overloads it might try to implicitly convert both
> StringRef and const char* to Twine and then add them since StringRef +
> const char * wouldn't otherwise be defined.
>
>   inline Twine operator+(const Twine &LHS, const Twine &RHS) {
>     return LHS.concat(RHS);
>   }
>
>   /// Additional overload to guarantee simplified codegen; this is
> equivalent to
>   /// concat().
>
>   inline Twine operator+(const char *LHS, const StringRef &RHS) {
>     return Twine(LHS, RHS);
>   }
>
>   /// Additional overload to guarantee simplified codegen; this is
> equivalent to
>   /// concat().
>
>   inline Twine operator+(const StringRef &LHS, const char *RHS) {
>     return Twine(LHS, RHS);
>   }
>
>
>
> ~Craig
>
>
> On Mon, Apr 29, 2019 at 3:18 PM Russell Wallace via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm looking at the documentation on Twine at
>> http://llvm.org/docs/ProgrammersManual.html#llvm-adt-twine-h and it
>> gives example code:
>>
>> void foo(const Twine &T);
>> ...
>> StringRef X = ...
>> unsigned i = ...
>> foo(X + "." + Twine(i));
>>
>> How exactly does that last line work? Since addition is left
associative,
>> I would expect it to be parsed as (X + ".") ... so it's
trying to add a
>> StringRef and a const char* before Twine can come into the picture at
all?
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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