Am Donnerstag, den 28.01.2021, 22:48 +0100 schrieb Martin Uecker:
.
>
> > > 3. And finally there is the problem that C++ does not
> > > have them.
> >
> > Once upon a time, the C++ committee spent quite some effort
> > on integrating VLAs.
> >
> > The history section in this paper
> > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0785r0.html
> > is probably a good start.
>
> Thanks.
I tried to look deeper on this, but to be honest I did not
fully understand why this went nowhere in C++.
To me VLA in C seem conceptually quite simple and powerful.
I use them a lot in numerical code and they are extremely
useful. There are theoretically sound (in contrast to some
other language aspects related to UB) and also reasonably
easy to implement in compilers (I did this myself last
year in my toy C compiler). So why not just take the
C feature as is?
It would be helpful if someone could summarize the problems
that make it difficult or undesirable to integrate this
fully into C++.
I wasn't around when this was discussed in the C++ committee, but I suspect these are some of the questions that need answers for VLAs to make it into C++:
template <typename T>
void v(T&& a) {
std::cout << typeid(T).name() << '\n';
}
template <typename T, size_t N>
void t(T (&a)[N]) {
std::cout << N << '\n';
}
void u(std::span<int> a) {
std::cout << a.size() << '\n';
}
int const len = 10;
void m(int n) {
int a[n]; // VLA
int b[len]; // normal array
std::array<char, sizeof(a)> a2; // ERROR: sizeof() is a runtime value on VLAs
std::array<char, sizeof(b)> b2; // OK
std::array<decltype(a), 5> a3; // ERROR: decltype() on VLA
std::array<decltype(b), 5> b3; // OK
v(a); // ERROR: no matching function for call to 'v(int [n])'
v(b); // OK
t(a); // ERROR: variable-sized array type 'long int' is not a valid template argument
t(b); // OK
u(a); // ERROR: could not convert '(int*)(& a)' from 'int*' to 'std::span<int>'
u(b); // OK
}
I think a more practical approach in C++ would be something like:
template <typename T>
std::span<T> alloca<T>(size_t n);
