c++20

Can a non-type template parameter be of type “void*”?

由 怎甘沉沦 提交于 2019-11-28 07:14:11
问题 Yakk - Adam Nevraumont said: Non-type template parameters of type void* are not allowed in at least some versions of the standard. Is this true? If it is true, in which versions of the standard are non-type template parameters of type void* not allowed? (Note: as noted in a comment to answer another comment, this is about non-type template parameters , not template type arguments , which can be any valid type-id per [temp.arg.type], including void* . 回答1: TL;DR Template parameters of type

How do I use C++ modules in Clang?

由 牧云@^-^@ 提交于 2019-11-28 06:46:50
Modules are an alternative to #includes. Clang has a complete implementation for C++ . How would I go about if I wanted to use modules using Clang now? Using import std.io; in a C++ source file does not work (compile) yet, as the specification for modules (which includes syntax) isn't final. The Clang documentation states that, when passing the -fmodules flag, #includes will be rewritten to their appropriate imports. However, checking the preprocessor suggests otherwise (test.cpp only contains #include <stdio.h> and an empty main): $ clang++-3.5 -fmodules -E test.cpp -o test $ grep " printf "

What is the <=> operator in C++?

由 耗尽温柔 提交于 2019-11-27 16:57:23
While I was trying to learn about C++ operators, I stumbled upon a strange comparison operator on cppreference.com , * in a table that looked like this: "Well, if these are common operators in C++, I better learn them", I thought. But all my attempts to elucidate this mystery were unsuccessful. Even here, on Stack Overflow I had no luck in my search. Is there a connection between <=> and C++ ? And if there is, what does this operator do exactly? * In the meantime cppreference.com updated that page and now contains information about the <=> operator. This is called the three-way comparison

Why aren't ranges' algorithms compatible with std's iterators?

由 别等时光非礼了梦想. 提交于 2019-11-27 16:14:30
问题 #include <vector> #include <iostream> #include <range/v3/all.hpp> int main() { auto coll = std::vector{ 1, 2, 3 }; ranges::copy( coll, ranges::ostream_iterator<int>{ std::cout, ", " } ); // ok ranges::copy( coll, std::ostream_iterator<int>{ std::cout, ", " } ); // error } The issue is shown in the code above. I use ranges-v3-0.3.7. To me, the generic algorithm copy shouldn't care about the destination iterator type as long as it meets the requirements of output iterator. If so, why aren't

How to use source_location in a variadic template function?

由 我怕爱的太早我们不能终老 提交于 2019-11-27 10:44:57
问题 The C++20 feature std::source_location is used to capture information about the context in which a function is called. When I try to use it with a variadic template function, I encountered a problem: I can't see a place to put the source_location parameter. The following doesn't work because variadic parameters have to be at the end: // doesn't work template <typename... Args> void debug(Args&&... args, const std::source_location& loc = std::source_location::current()); The following doesn't

Will specialization of function templates in std for program-defined types no longer be allowed in C++20?

由 安稳与你 提交于 2019-11-27 02:33:33
问题 Quote from cppreference.com: Adding template specializations It is allowed to add template specializations for any standard library |class (since C++20)| template to the namespace std only if the declaration depends on at least one program-defined type and the specialization satisfies all requirements for the original template, except where such specializations are prohibited. Does it mean, that starting from C++20, adding specializations of function templates to the std namespace for user

How do I use C++ modules in Clang?

由 淺唱寂寞╮ 提交于 2019-11-27 01:35:42
问题 Modules are an alternative to #includes. Clang has a complete implementation for C++. How would I go about if I wanted to use modules using Clang now? Using import std.io; in a C++ source file does not work (compile) yet, as the specification for modules (which includes syntax) isn't final. The Clang documentation states that, when passing the -fmodules flag, #includes will be rewritten to their appropriate imports. However, checking the preprocessor suggests otherwise (test.cpp only contains

Why does C++11 have implicit moves for value parameters, but not for rvalue parameters?

由 自闭症网瘾萝莉.ら 提交于 2019-11-27 01:06:33
问题 In C++11, value parameters (and other values) enjoy implicit move when returned: A func(A a) { return a; // uses A::A(A&&) if it exists } At least in MSVC 2010, rvalue reference parameters need std::move : A func(A && a) { return a; // uses A::A(A const&) even if A::A(A&&) exists } I would imagine that inside functions, an rvalue reference and a value behave similar, with the only difference that in case of values, the function itself is responsible for destruction, while for rvalue

Does constraint subsumption only apply to concepts?

由 只愿长相守 提交于 2019-11-26 20:37:36
问题 Consider this example: template <typename T> inline constexpr bool C1 = true; template <typename T> inline constexpr bool C2 = true; template <typename T> requires C1<T> && C2<T> constexpr int foo() { return 0; } template <typename T> requires C1<T> constexpr int foo() { return 1; } constexpr int bar() { return foo<int>(); } Is the call foo<int>() ambiguous, or does the constraint C1<T> && C2<T> subsume C1<T> ? 回答1: Yes. Only concepts can be subsumed. The call to foo<int> is ambiguous because

What is the <=> operator in C++?

由 自古美人都是妖i 提交于 2019-11-26 18:47:52
问题 While I was trying to learn about C++ operators, I stumbled upon a strange comparison operator on cppreference.com, * in a table that looked like this: "Well, if these are common operators in C++, I better learn them", I thought. But all my attempts to elucidate this mystery were unsuccessful. Even here, on Stack Overflow I had no luck in my search. Is there a connection between <=> and C++ ? And if there is, what does this operator do exactly? * In the meantime cppreference.com updated that
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