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Re: [std-proposals] A new kind variable for immediate functions

From: Marcin Jaczewski <marcinjaczewski86_at_[hidden]>
Date: Fri, 4 Mar 2022 13:30:37 +0100
pt., 4 mar 2022 o 12:54 Torben Thaysen via Std-Proposals
<std-proposals_at_[hidden]> napisaƂ(a):
> Currently when computing things at compile time one sometimes faces a problem when trying to output the result to a constexpr variable.
> This happens when the result is data structure without a fixed size that utilizes dynamic memory allocation, which in itself is not a
> constant expression (for example a vector). Today the result can be converted to a more appropriate fixed sized structure like this:
> constexpr std::vector<int> compute_vector();
> constexpr std::size_t extract_size() { return compute_vector().size(); }
> constexpr auto as_array() {
> auto vec = compute_vector();
> std::array<int, extract_size()> arr;
> std::copy(vec.begin(), vec.end(), arr.begin());
> return arr;
> }
> But this somewhat awkwardly involves doing the computation twice. The only way around this is to copy the data to a large intermediate array
> then copy it again into an array of the right size. Neither of these solutions is really ideal and that is what this proposal aims to solve.
> For this I want to introduce a new kind of variable that can be declared inside an immediate function. With this the above example could look like:
> constexpr std::vector<int> compute_vector();
> consteval auto as_array() {
> consteval auto vec = compute_vector(); // example syntax
> std::array<int, vec.size()> arr;
> std::copy(vec.begin(), vec.end(), arr.begin());
> return arr;
> }
> The requirements for this type of variable should conceptually be similar to those for constexpr variables (see and 7.7.11) except that:
> Targets of pointers and references don't have to have static storage duration. In particular pointers to dynamically allocated memory are allowed.
> Memory allocations performed in the initialization can (and must) be deallocated in the destructor.
> And (i think) pointers to immediate functions can also be allowed, since this type of variable only exists inside immediate functions. Although this
> isn't required for the aim of this proposal.
> To my reading this lifts all the requirements of 7.7.11 so that any core constant expression with the modified allocation requirement would be allowed.
> Like with constexpr variables these new variables should also be implied to be const. And I propose for consistency they should be allowed to be
> declared in any immediate function context as by 7.7.13 (i.e. also inside a if consteval statement).
> Looking forward to your feedback
> Torben
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

I think this should be fixed a bit diffrent way. Current `constexpr`
variables can materialize in result code and this is problem if we use
memory allocation.
One way I could see to fix it make option for "pure" `constexpr
variables that only live during compilation and are not accessible in
anyway by run time.
This way we could make:
int main() //can be used in any function!
    consteval auto s1 = std::string("Test"); //variable and memory
allocation live only during "compilation" of `main`, we could consider
this a new `static` in some way.
    static constexpr std::array<int, s1.size()> a1 = { s1.begin(),
s1.end() }; //assume that array have constructor that can copy from

    auto s2 = std::string(s1); //Error! `s1` is not accessible here!
    auto s3 = std::string(a1.begin(), a1.end()); //normal string that
copy data from static memory

Received on 2022-03-04 12:30:50