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::formatCalendartimezonestd::source_locationstd::span航天飞机运算符 <=>std::endian判断大小端std::remove_cvrefbind_frontstd::atomic_refstd::map<Key,T,Compare,Allocator>::containsstd::barrierstd::latch锁存器std::counting_semaphorestring::starts_with / ends_withstd::sizestd::is_bounded_array_v和std::is_unbounded_arraystd::erase_ifMathematical constantsstd::midpointstd::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 variables int num = 42; std::string name = "John"; // Use std::format to format a string with placeholders // for variables std::string formatted_str = std::format( "My name is {} and my favorite number is {}", name, num); // Print formatted string to console std::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 precision auto now = zoned_time{current_zone(), system_clock::now()}.get_local_time(); // Get a local time_point with days precision auto ld = floor<days>(now); // Convert local days-precision time_point to a local {y, m, d} calendar year_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"; else std::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"; else std::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 here static 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 atomic sync_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 statestd::binary_semaphore smphSignalMainToThread{0}, smphSignalThreadToMain{0}; void ThreadProc(){ // wait for a signal from the main proc // by attempting to decrement the semaphore smphSignalMainToThread.acquire(); // this call blocks until the semaphore's count // is increased from the main proc std::cout << "[thread] Got the signal\n"; // response message // wait for 3 seconds to imitate some work // being done by the thread using namespace std::literals; std::this_thread::sleep_for(3s); std::cout << "[thread] Send the signal\n"; // message // signal the main proc back smphSignalThreadToMain.release();} int main(){ // create some worker thread std::thread thrWorker(ThreadProc); std::cout << "[main] Send the signal\n"; // message // signal the worker thread to start working // by increasing the semaphore's count smphSignalMainToThread.release(); // wait until the worker thread is done doing the work // by attempting to decrement the semaphore's count smphSignalThreadToMain.acquire(); std::cout << "[main] Got the signal\n"; // response message thrWorker.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 available auto 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 -inf std::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';}版权声明：内容来源于互联网和用户投稿 如有侵权请联系删除