How to implement dangling-pointer warning in custom string type

Question

The following code is invalid because it takes a pointer into a temporary object (triggering -Wdangling-gsl):

static std::string f() {
    return "hi";
}

void func() {
    const char* ptr = f().c_str();
}

<source>:8:23: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling-gsl]

I have a custom string class which looks and feels a lot like std::string, but internally represents the text in a different way.

Is there a way to make MyString::c_str also generate a similar warning if used in this way?

Interesting question. I'd take a peek a your <string> header file and see what you see, — john
– john, Commented Jul 10, 2023 at 15:07
This question is compiler specific (as much as I know C++ language does not support this), so it would be helpful to specify which one you are using. — Slava
– Slava, Commented Jul 10, 2023 at 15:09
For it to be universal it has to be supported on the language level, and it is obviously not supported. — Slava
– Slava, Commented Jul 10, 2023 at 15:16
There's nothing that stands out in libc++. _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const value_type* c_str() const _NOEXCEPT {return data();} I checked the call chain and there didn't seem to be any special attributes for this. — StilesCrisis
– StilesCrisis, Commented Jul 10, 2023 at 15:39

Jarod42 · Accepted Answer · 2023-07-10 15:16:57Z

0

I don't think so for warning (unless compiler has extension for that), but you can forbid that function on temporary (which would forbid also valid usage).

const char* MyString::c_str() const && = delete;

edited Jul 10, 2023 at 15:16

answered Jul 10, 2023 at 15:08

Jarod42

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3 Comments

StilesCrisis Over a year ago

This would forbid valid usage as well, like printf("%s", foo().c_str())

Slava Over a year ago

@StilesCrisis +1 - ideone.com/IRtALB

Jarod42 Over a year ago

@StilesCrisis: I agree. C++, contrary to Rust doesn't have lifetime within the type to spot dangling pointer.

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