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by scintill76 677 days ago
> It will even print std::string’s correctly if you cast them to const char* by hand

What does that mean? I think `print str.c_str()` has worked for me in GDB before, but sounds like you did something different.

1 comments

I was observing that `p (const char *)str` also worked in my experiment, but I’m far from a C++ expert and upon double-checking this seems to have been more of an accident than intended behaviour, because there is no operator const_pointer in basic_string that I can find. Definitely use `p str.c_str()`.
If your std::string was using a short string optimization, that would explain the “accident”.

Some implementations even put char[0] at the first byte in the optimized form.

That explanation doesn't work IMO, unless `str` is a std::string pointer, which is contrary to the syntax GP suggested with `str.c_str()`.

It doesn't seem possible in actual C++ that the cast from non-pointer to pointer would work at all (even if a small string happens to be inlined at offset 0.) Like GP, I looked for a conversion operator, and I don't think it's there. Maybe it is a feature of the gdb parser.

Good point, but if it’s a long string, 2/3 of the most common implementations would make the first word the c_str()-equivalent pointer:

https://devblogs.microsoft.com/oldnewthing/20240510-00/?p=10...

So it's actually printing *(const char **)&s?
The first pointer-sized chunk of the string structure is a pointer to the C-string representation. So the cast works as written.