Why is an attribute needed for this? Why not rely on static analysis tools?

Jeremy

On Tue, Jul 22, 2025 at 06:05 Frederick Virchanza Gotham via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Sometimes we want the return value from a function not only to not be
> discarded, but more specifically we want it to be stored inside a
> local variable.
>
> Consider a method like std::synchronized_value::rlock, which returns
> by value a type similar to "std::lock_guard". We always want this
> return value to be stored in a variable:
>
> auto mylock = mysyncvalue.wlock();
>
> If the programmer is only going to call one method on it, then they
> should instead use "operator->", therefore we can consider the
> following to be incorrect:
>
> mysyncvalue.wlock()->DoSomething();
>
> and instead it should be replaced with:
>
> mysyncvalue->DoSomething();
>
> Therefore, it would make sense to mark the 'wlock' method as
> [[must_store]].
>
> Another example is the wxWidgets library, with the class wxString. The
> method, "wxString::c_str" looks like this:
>
> wxCStrData wxString::c_str() const
> {
> return wxCStrData(this);
> }
>
> When you invoke "c_str" on a wxString object, it returns a wxCStrData
> object. The wxCStrData class has two implicit conversions:
>
> operator const wchar_t*() const { return AsWChar(); }
> operator const char*() const { return AsChar(); }
>
> If you build the wxWidgets library in Unicode mode, then the method
> "AsWChar" returns a pointer to a pre-existing array. However if you
> build the wxWidgets library in ASCII mode, and then invoke "AsWChar",
> it dynamically allocates a temporary array and gives you the pointer.
> This could allow us to put a very interesting bug into our code:
>
> wxString mystr( wxS("Hello I'm a monkey") );
>
> wchar_t *const p = mystr.c_str();
>
> wcout << p << endl;
>
> The above code will work fine on Unicode, but might crash on ASCII.
> The dynamically-allocated array gets destroyed when "c_str()" returns.
> The solution is to do this:
>
> wxString mystr("Hello I'm a monkey");
>
> auto const converter = mystr.c_str();
> wchar_t *const p = converter;
>
> wcout << p << endl;
>
> If we can mark a function as [[must_store]] then it will prevent stuff
> like this from making its way into the release build.
> --
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