tip-of-the-week / cpp Goto Github PK

Hey,
these tips of the week look really interesting and informative, thanks for sharing them.
But I am not sure if it is correct what you write in Tip190. I think std::ostream_joiner did not get added to C++20, or did it?

Regards,
Kilian

there have a bug in cpp_tip_of_the_week/README.md:15.
Thank you for your weekly updates。

In 315.md the first proposed solution is

template<class T, class... Ts>
constexpr auto count_compatible = (std::is_layout_compatible_v<T, Ts> + ... + std::size_t{});

but the tests pass without + std::size_t{}.

template<class T, class... Ts>
constexpr auto count_compatible = (std::is_layout_compatible_v<T, Ts> + ...);

I think this is due to C++ being weird (aka have automatic conversions)

constexpr bool b1 = true;
constexpr bool b2 = true;
constexpr auto test = b1 + b2;
static_assert(2 == test);
static_assert(std::is_same_v<decltype(test), const int>);

The introduction of size_t might help for very large type lists, but it probably should go first. Not sure though.

template<class T, class... Ts>
constexpr auto count_compatible = (std::size_t{} + ... + std::is_layout_compatible_v<T, Ts>);

Am I missing something?

template<class... Ts, class T>
void foo(Ts..., T t) { return t; }
int main() {
  std::cout << foo(1); // prints 1
  std::cout << foo(1, 2, 3, 4); // prints 4
  std::cout << foo(); // compilation error
}

https://godbolt.org/z/boxjzq

The function foo's return type is void, so std::cout will not get a value from foo.

In the compiler explore, the foo(1, 2, 3, 4) also gets an error.

<source>:8:16: error: no matching function for call to 'foo'
  std::cout << foo(1, 2, 3, 4); // prints 4
               ^~~
<source>:4:3: note: candidate function [with Ts = <>, T = int] not viable: requires single argument 't', but 4 arguments were provided
T foo(Ts..., T t) { return t; }
  ^

Is this what the tip want?

I rewrite the function foo like below.

template <class ... Ts, class T>
T foo(T t, Ts ...) { return t; }

The second call will print 1 instead of 4.

So I also want to know how to write a function that looks like the one in tip 182 and gets no error when prints 4.

Maybe I will use function like below:

template <class T>
T foo(T t) { return t;}

template <class ... Ts, class T>
T foo(T t, Ts... args) { return foo(args ...); }