On Sun, Apr 27, 2025 at 11:27 AM Frederick Virchanza Gotham via
Std-Proposals <std-proposals_at_[hidden]> wrote:
>
>
>
> On Sunday, April 27, 2025, Jason McKesson wrote:
>>
>>
>>
>> No. There is nothing fundamentally wrong with calling a function that
>> *might* throw exceptions from one that won't.
>>
>> You have no idea what could cause the possibly throwing function to
>> throw. Maybe there are certain parameters that would cause it to throw
>> (nullptr, index-out-of-bounds, etc), but the non-throwing function
>> already did the check and no throwing can ever happen.
>>
>> Unless you *know* for certain that this is not the case, you have no
>> right to decide for other people that this code is ill-formed.
>
>
>
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> Let's say a 3rd party library has a function as follows:
>
> bool RegisterMatrix(unsigned x, unsigned y) noexcept(false);
>
> And let's say that the instruction manual says:
>
> Throws: An object of type 'dimensions_invalid' if 'y' is not a multiple of 'x'.
>
> And then in your own code you invoke this function as follows:
>
> bool RecordAdd(unsigned const x) noexcept
> {
> return RegisterMatrix(x, x * 4u);
> }
>
> In this instance, we know that RegisterMatrix definitely won't throw an exception because we set the 'y' argument equal to 'four times X'. So now let's try force a compiler error with 'noexcept(explicit)' as follows:
>
> bool RecordAdd(unsigned const x) noexcept(explicit)
> {
> return RegisterMatrix(x, x * 4u);
> }
>
> Now the compiler refuses to compile the code because it sees a 'noexcept(false)' function called from a 'noexcept(true)' function. Here we need to tell the compiler that RegisterMatrix won't throw, perhaps by using 'std::won't_throw' as follows:
>
> bool RecordAdd(unsigned const x) noexcept(explicit)
> {
> return std::wont_throw( RegisterMatrix, x, x * 4u );
> }

All you did was just re-describe *what* you want. I fully and
completely understand what you want.

I'm saying that what you want *is bad*. You should not *have* to code like this.