using for Enumeration Values
对比一下 C++20 前后的区别：
enum class State { open, progress, done = 9};// Before C++20void print(State s) { switch (s) { case State::open: std::cout << "open\n"; break; case State::done: std::cout << "done\n"; break; case State::progress: std::cout << "progress\n"; break; default: assert(nullptr); }}
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// Since C++20void print1(State s) { using enum State; switch (s) { case open: std::cout << "open\n"; break; case done: std::cout << "done\n"; break; case progress: std::cout << "progress\n"; break; default: assert(nullptr); }}
void print2(State s) { using State::open, State::done, State::progress; switch (s) { case open: std::cout << "open\n"; break; case done: std::cout << "done\n"; break; case progress: std::cout << "progress\n"; break; default: assert(nullptr); }}
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Range-Based for Loop with Initialization
基于范围的 for 循环是 C++11 引入的新特性，自 C++20 起，基于范围循环也可以进行初始化。
int main() { std::vector v{1, 2, 3}; for (int i{1}; const auto& item : v) std::cout << std::format("{}: {}\n", i++, item);}
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Feature Test Macros
特性测试宏，正如其名，是为了当前版本编译器是否支持某个语言特性。这个宏以__cpp 为前缀。
#if __cpp_generic_lambdas >= 201707 // generic lambdas with template parameters can be used#endif
#ifndef __cpp_lib_as_consttemplate <typename T>const T& as_const(T& t) { return t;}#endif
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Attribute [[no_unique_address]]
#include <iostream>
struct Empty {};
struct I { int i;};
struct S { Empty e; int i;};
int main() { std::cout << "sizeof(Empty): " << sizeof(Empty) << '\n'; std::cout << "sizeof(I): " << sizeof(I) << '\n'; std::cout << "sizeof(S): " << sizeof(S) << '\n';}
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空类为了区分不同对象的地址，字节大小是 1；而结构体 S 由于内存对齐的原因，所以字节大小是 8。输出结果毫无疑问是 1，4，8。
#include <iostream>
struct Empty {};
// EBCOstruct S : Empty { int i;};
// no_unique_addressstruct S2 { [[no_unique_address]] Empty e; int i;};
int main() { std::cout << "sizeof(Empty): " << sizeof(Empty) << '\n'; std::cout << "sizeof(S): " << sizeof(S) << '\n'; std::cout << "sizeof(S2): " << sizeof(S2) << '\n';}
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注解标签 no_unique_address 能起到和空基类优化相同的效果。
Attributes [[likely]] and [[unlikely]]
在 if/else，switch 分支当中都可以使用，帮助编译器作分支预测的优化。
int f(int n) { if (n <= 0) [[unlikely]] { return n; } else { return n * n; }}
int g(int n) { if (n <= 0) { return n; } else [[likely]] { return n * n; }}
int h(int n) { switch (n) { case 1: // break; [[likely]] case 2: // break; }
return 0;}
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Attribute [[nodiscard]] with Parameter
nodiscard 用于修饰函数，当被修饰的函数发生调用（仅仅调用而不作赋值或者强制转换操作），编译器会报警告信息。
[[nodiscard]]int f(int n) { return n;}
int main() { f(1); // Warning int n = f(1); // OK reinterpret_cast<int *>(f(1)); // OK}
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New Character Type char8_t
using namespace std::literals;
auto c = u8'c'; // char8_tauto s = u8"Hello World"; // const char8_t *auto str1 = u8"Hello World"s; // std::u8stringauto str2 = u8"Hello World"sv; // std::u8string_view
std::cout << u8'c' << '\n'; // OK in C++17, error in C++20std::cout << u8"Hello World\n"; // OK in C++17, error in C++20std::cout << u8"Hello World\n"s; // OK in C++17, error in C++20std::cout << u8"Hello World\n"sv; // OK in C++17, error in C++20
std::cout << c << '\n'; // OK in C++17, error in C++20std::cout << s << '\n'; // OK in C++ 17, error in C++20std::cout << str1 << '\n'; // OK in C++17, error in C++20std::cout << str2 << '\n'; // OK in C++17, error in C++20
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文章转载自：ChebyshevTST
原文链接：https://www.cnblogs.com/ChebyshevTST/p/17868410.html
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