c++20的新增的小的特性
c++20新增的特性非常多，其中concept，coroutine，module和range为四大特性，在之前的章节中已有涉及，本文则对其他的一些小改动进行讲解。
- c++20的新增的小的特性
- std::format
- Calendar
- timezone
- std::source_location
- std::span
- 航天飞机运算符 <=>
- std::endian判断大小端
- std::remove_cvref
- bind_front
- std::atomic_ref
std::map<Key,T,Compare,Allocator>::contains- std::barrier
- std::latch锁存器
- std::counting_semaphore
- string::starts_with / ends_with
- std::size
- std::is_bounded_array_v和std::is_unbounded_array
- std::erase_if
- Mathematical constants
- std::midpoint
- std::lerp
std::format
c++20支持新的字符串格式化方式std::format
#include <format>
#include <iostream>using namespace std;int main()
{// Declare variablesint num = 42;std::string name = "John";// Use std::format to format a string with placeholders// for variablesstd::string formatted_str = std::format("My name is {} and my favorite number is {}", name,num);// Print formatted string to consolestd::cout << formatted_str << std::endl;return 0;
}
Calendar
#include <chrono>int main()
{using namespace std::chrono;// Get a local time_point with system_clock::duration precisionauto now = zoned_time{current_zone(), system_clock::now()}.get_local_time();// Get a local time_point with days precisionauto ld = floor<days>(now);// Convert local days-precision time_point to a local {y, m, d} calendaryear_month_day ymd{ld};// Split time since local midnight into {h, m, s, subseconds}hh_mm_ss hms{now - ld};// This part not recommended. Stay within the chrono type system.int year{ymd.year()};int month = unsigned{ymd.month()};int day = unsigned{ymd.day()};int hour = hms.hours().count();int minute = hms.minutes().count();int second = hms.seconds().count();
}
timezone
时区工具
#include <chrono>
#include <iostream>int main() {const std::chrono::zoned_time cur_time{ std::chrono::current_zone(),std::chrono::system_clock::now() };std::cout << cur_time << '\n';
}
std::source_location
文件位置的工具
#include <iostream>
#include <source_location>
#include <string_view>void log(const std::string_view message,const std::source_location location =std::source_location::current())
{std::clog << "file: "<< location.file_name() << '('<< location.line() << ':'<< location.column() << ") `"<< location.function_name() << "`: "<< message << '\n';
}template<typename T>
void fun(T x)
{log(x);
}int main(int, char*[])
{log("Hello world!");fun("Hello C++20!");
}
std::span
过去如果一个函数想接受无法确定数组长度的数组作为参数，那么一定需要声明两个参数：数组指针和长度：
void set_data(int *arr, int len) {}int main()
{int buf[128]{ 0 };set_data(buf, 128);
}
这种人工输入增加了编码的风险，数组长度的错误输入会引发程序的未定义行为，甚至是成为可被利用的漏洞。C++20标准库为我们提供了一个很好解决方案std::span，通过它可以定义一个基于连续序列对象的视图，包括原生数组，并且保留连续序列对象的大小。例如：
#include <iostream>
#include <span>
void set_data(std::span<int> arr) {std::cout << arr.size();
}int main()
{int buf[128]{ 0 };set_data(buf);
}
航天飞机运算符 <=>
(a <=> b) < 0 if a < b,
(a <=> b) > 0 if a > b,
(a <=> b) == 0 if a and b are equal/equivalent.
#include <compare>
#include <iostream>int main()
{double foo = -0.0;double bar = 0.0;auto res = foo <=> bar;if (res < 0)std::cout << "-0 is less than 0";else if (res > 0)std::cout << "-0 is greater than 0";else if (res == 0)std::cout << "-0 and 0 are equal";elsestd::cout << "-0 and 0 are unordered";
}
std::endian判断大小端
#include <bit>
#include <iostream>int main()
{if constexpr (std::endian::native == std::endian::big)std::cout << "big-endian\n";else if constexpr (std::endian::native == std::endian::little)std::cout << "little-endian\n";elsestd::cout << "mixed-endian\n";
}
std::remove_cvref
去除cv和引用
#include <type_traits>int main()
{static_assert(std::is_same_v<std::remove_cvref_t<int>, int>);static_assert(std::is_same_v<std::remove_cvref_t<int&>, int>);static_assert(std::is_same_v<std::remove_cvref_t<int&&>, int>);static_assert(std::is_same_v<std::remove_cvref_t<const int&>, int>);static_assert(std::is_same_v<std::remove_cvref_t<const int[2]>, int[2]>);static_assert(std::is_same_v<std::remove_cvref_t<const int(&)[2]>, int[2]>);static_assert(std::is_same_v<std::remove_cvref_t<int(int)>, int(int)>);
}
bind_front
和std::bind是一个系列的方法，bind_front可以绑定前n个参数而不用敲placeholder。
#include <functional>
#include <iostream>int main()
{auto calc=[](int a, int b, int c) { return a+b-c;};auto aa = std::bind_front(calc, 1,2);std::cout << aa (3)<<"\n";auto bb = std::bind_front(calc, 1,2,3);std::cout << bb ()<<"\n";auto cc=std::bind(calc, 1,std::placeholders::_2,std::placeholders::_1);std::cout<<cc(3,2)<<"\n"; auto dd=std::bind(calc, std::placeholders::_1,std::placeholders::_2,3);std::cout<<dd(1,2);
}
std::atomic_ref
原子引用
在下面的例子中，最终将打印100
#include <atomic>
#include <thread>
#include <vector>
#include <iostream>int do_count(int value)
{std::atomic<int> counter { value };std::vector<std::thread> threads;for (int i = 0; i < 10; ++i){threads.emplace_back([&counter]() {for (int i = 0; i < 10; ++i){++counter;{using namespace std::chrono_literals;std::this_thread::sleep_for(50ms);}}});}for (auto& t : threads) t.join();return counter;
}int main()
{int result = do_count(0);std::cout << result << '\n'; // prints 100
}
std::atomic并作用于引用不生效， 下面的例子将打印0。
#include <atomic>
#include <thread>
#include <vector>
#include <iostream>void do_count_ref(int& value)
{std::atomic<int> counter{ value };std::vector<std::thread> threads;for (int i = 0; i < 10; ++i){threads.emplace_back([&counter]() {for (int i = 0; i < 10; ++i){++counter;{using namespace std::chrono_literals;std::this_thread::sleep_for(50ms);}}});}for (auto& t : threads) t.join();
}int main()
{int value = 0;do_count_ref(value);std::cout << value << '\n'; // prints 0
}
使用atimic_ref进行修改，可以打印100。
#include <atomic>
#include <thread>
#include <vector>
#include <iostream>void do_count_ref(int& value)
{std::atomic_ref<int> counter{ value };std::vector<std::thread> threads;for (int i = 0; i < 10; ++i){threads.emplace_back([&counter]() {for (int i = 0; i < 10; ++i){++counter;{using namespace std::chrono_literals;std::this_thread::sleep_for(50ms);}}});}for (auto& t : threads) t.join();
}int main()
{int value = 0;do_count_ref(value);std::cout << value << '\n'; // prints 0
}
std::map<Key,T,Compare,Allocator>::contains
map新增contains方法，在此之前使用的是find或者count。
#include <iostream>
#include <map>int main()
{std::map<int,char> example = {{1,'a'},{2,'b'}};for(int x: {2, 5}) {if(example.contains(x)) {std::cout << x << ": Found\n";} else {std::cout << x << ": Not found\n";}}
}
std::barrier
线程屏障
#include <barrier>
#include <iostream>
#include <string>
#include <thread>
#include <vector>int main()
{const auto workers = { "Anil", "Busara", "Carl" };auto on_completion = []() noexcept{// locking not needed herestatic auto phase = "... done\n" "Cleaning up...\n";std::cout << phase;phase = "... done\n";};std::barrier sync_point(std::ssize(workers), on_completion);auto work = [&](std::string name){std::string product = " " + name + " worked\n";std::cout << product; // ok, op<< call is atomicsync_point.arrive_and_wait();product = " " + name + " cleaned\n";std::cout << product;sync_point.arrive_and_wait();};std::cout << "Starting...\n";std::vector<std::jthread> threads;threads.reserve(std::size(workers));for (auto const& worker : workers)threads.emplace_back(work, worker);
}
std::latch锁存器
latch = single-use barrier.
#include <functional>
#include <iostream>
#include <latch>
#include <string>
#include <thread>int main() {struct job {const std::string name;std::string product{"not worked"};std::thread action{};} jobs[] = {{"annika"}, {"buru"}, {"chuck"}};std::latch work_done{std::size(jobs)};std::latch start_clean_up{1};auto work = [&](job& my_job) {my_job.product = my_job.name + " worked";work_done.count_down();start_clean_up.wait();my_job.product = my_job.name + " cleaned";};std::cout << "Work starting... ";for (auto& job : jobs) {job.action = std::thread{work, std::ref(job)};}work_done.wait();std::cout << "done:\n";for (auto const& job : jobs) {std::cout << " " << job.product << '\n';}std::cout << "Workers cleaning up... ";start_clean_up.count_down();for (auto& job : jobs) {job.action.join();}std::cout << "done:\n";for (auto const& job : jobs) {std::cout << " " << job.product << '\n';}
}
std::counting_semaphore
计数信号量
#include <chrono>
#include <iostream>
#include <semaphore>
#include <thread>// global binary semaphore instances
// object counts are set to zero
// objects are in non-signaled state
std::binary_semaphoresmphSignalMainToThread{0},smphSignalThreadToMain{0};void ThreadProc()
{// wait for a signal from the main proc// by attempting to decrement the semaphoresmphSignalMainToThread.acquire();// this call blocks until the semaphore's count// is increased from the main procstd::cout << "[thread] Got the signal\n"; // response message// wait for 3 seconds to imitate some work// being done by the threadusing namespace std::literals;std::this_thread::sleep_for(3s);std::cout << "[thread] Send the signal\n"; // message// signal the main proc backsmphSignalThreadToMain.release();
}int main()
{// create some worker threadstd::thread thrWorker(ThreadProc);std::cout << "[main] Send the signal\n"; // message// signal the worker thread to start working// by increasing the semaphore's countsmphSignalMainToThread.release();// wait until the worker thread is done doing the work// by attempting to decrement the semaphore's countsmphSignalThreadToMain.acquire();std::cout << "[main] Got the signal\n"; // response messagethrWorker.join();
}
string::starts_with / ends_with
字符串开头/结尾
#include <iostream>
#include <string>
#include <string_view>template<typename PrefixType>
void test_prefix_print(const std::string& str, PrefixType prefix)
{std::cout << '\'' << str << "' starts with '" << prefix << "': "<< str.starts_with(prefix) << '\n';
}int main()
{std::boolalpha(std::cout);auto helloWorld = std::string("hello world");test_prefix_print(helloWorld, std::string_view("hello"));test_prefix_print(helloWorld, std::string_view("goodbye"));test_prefix_print(helloWorld, 'h');test_prefix_print(helloWorld, 'x');
}
std::size
size的common方法
#include <iostream>
#include <vector>int main()
{std::vector<int> v{3, 1, 4};std::cout << std::size(v) << '\n';int a[]{-5, 10, 15};std::cout << std::size(a) << '\n';// since C++20 the signed size (ssize) is availableauto i = std::ssize(v);for (--i; i != -1; --i)std::cout << v[i] << (i ? ' ' : '\n');std::cout << "i = " << i << '\n';
}
std::is_bounded_array_v和std::is_unbounded_array
#include <iostream>
#include <type_traits>#define OUT(...) std::cout << #__VA_ARGS__ << " : " << __VA_ARGS__ << '\n'class A {};int main()
{std::cout << std::boolalpha;OUT(std::is_bounded_array_v<A>);OUT(std::is_bounded_array_v<A[]>);OUT(std::is_bounded_array_v<A[3]>);OUT(std::is_bounded_array_v<float>);OUT(std::is_bounded_array_v<int>);OUT(std::is_bounded_array_v<int[]>);OUT(std::is_bounded_array_v<int[3]>);
}
#include <iostream>
#include <type_traits>#define OUT(...) std::cout << #__VA_ARGS__ << " : " << __VA_ARGS__ << '\n'class A {};int main()
{std::cout << std::boolalpha;OUT( std::is_unbounded_array_v<A> );OUT( std::is_unbounded_array_v<A[]> );OUT( std::is_unbounded_array_v<A[3]> );OUT( std::is_unbounded_array_v<float> );OUT( std::is_unbounded_array_v<int> );OUT( std::is_unbounded_array_v<int[]> );OUT( std::is_unbounded_array_v<int[3]> );
}
std::erase_if
按照条件erase数据
#include <iostream>
#include <numeric>
#include <string_view>
#include <vector>void print_container(std::string_view comment, const std::vector<char>& c)
{std::cout << comment << "{ ";for (auto x : c)std::cout << x << ' ';std::cout << "}\n";
}int main()
{std::vector<char> cnt(10);std::iota(cnt.begin(), cnt.end(), '0');print_container("Initially, cnt = ", cnt);std::erase(cnt, '3');print_container("After erase '3', cnt = ", cnt);auto erased = std::erase_if(cnt, [](char x) { return (x - '0') % 2 == 0; });print_container("After erase all even numbers, cnt = ", cnt);std::cout << "Erased even numbers: " << erased << '\n';
}
对比之前的remove_if和erase
#include <iostream>
#include <vector>
#include <algorithm>bool isEven(int n) // 是否是偶数
{return n % 2 == 0;
}int main()
{std::vector<int> vecTest;for (int i = 0; i < 10; ++i)vecTest.push_back(i);for (int i = 0; i < vecTest.size(); ++i)std::cout << vecTest[i] << " ";std::cout << std::endl;// 移动元素std::vector<int>::iterator itor = std::remove_if(vecTest.begin(), vecTest.end(), isEven);// 查看移动后的变化for (int i = 0; i < vecTest.size(); ++i)std::cout << vecTest[i] << " ";std::cout << std::endl;// 删除元素vecTest.erase(itor, vecTest.end());for (int i = 0; i < vecTest.size(); ++i)std::cout << vecTest[i] << " ";return 0;
}Mathematical constants
c++20新增了一些数学常量
#include <numbers>
#include <iostream>int main() {std::cout << std::numbers::log2e_v<double> << std::endl;
}
std::midpoint
#include <cstdint>
#include <iostream>
#include <limits>
#include <numeric>int main()
{std::uint32_t a = std::numeric_limits<std::uint32_t>::max();std::uint32_t b = std::numeric_limits<std::uint32_t>::max() - 2;std::cout << "a: " << a << '\n'<< "b: " << b << '\n'<< "Incorrect (overflow and wrapping): " << (a + b) / 2 << '\n'<< "Correct: " << std::midpoint(a, b) << "\n\n";auto on_pointers = [](int i, int j){char const* text = "0123456789";char const* p = text + i;char const* q = text + j;std::cout << "std::midpoint('" << *p << "', '" << *q << "'): '"<< *std::midpoint(p, q) << "'\n";};on_pointers(2, 4);on_pointers(2, 5);on_pointers(5, 2);on_pointers(2, 6);
}
std::lerp
线性计算
a+t(b-a)
#include <cassert>
#include <cmath>
#include <iostream>float naive_lerp(float a, float b, float t)
{return a + t * (b - a);
}int main()
{std::cout << std::boolalpha;const float a = 1e8f, b = 1.0f;const float midpoint = std::lerp(a, b, 0.5f);std::cout << "a = " << a << ", " << "b = " << b << '\n'<< "midpoint = " << midpoint << '\n';std::cout << "std::lerp is exact: "<< (a == std::lerp(a, b, 0.0f)) << ' '<< (b == std::lerp(a, b, 1.0f)) << '\n';std::cout << "naive_lerp is exact: "<< (a == naive_lerp(a, b, 0.0f)) << ' '<< (b == naive_lerp(a, b, 1.0f)) << '\n';std::cout << "std::lerp(a, b, 1.0f) = " << std::lerp(a, b, 1.0f) << '\n'<< "naive_lerp(a, b, 1.0f) = " << naive_lerp(a, b, 1.0f) << '\n';assert(not std::isnan(std::lerp(a, b, INFINITY))); // lerp here can be -infstd::cout << "Extrapolation demo, given std::lerp(5, 10, t):\n";for (auto t{-2.0}; t <= 2.0; t += 0.5)std::cout << std::lerp(5.0, 10.0, t) << ' ';std::cout << '\n';
}
