C++现代教程五

#pragma once _Pragma(once) # C/C混合编程 #ifdef __cplusplus extern C { #endif // 一些c代码 #ifdef __cplusplus } #endif # 继承构造 struct A { A(int i) {} A(double d,int i){} A(float f,int i,const char* c){} //...等等系列的构造函数版本 } old: struct B:A { B(int i):A(i){} B(double d,int i):A(d,i){} B(folat f,int i,const char* c):A(f,i,e){} //......等等好多个和基类构造函数对应的构造函数 } new: struct B:A { using A::A; //关于基类各构造函数的继承一句话搞定 //...... } #regex std::regex base_regex(([a-z])\\.txt); std::smatch base_match; for(const auto fname: fnames) { if (std::regex_match(fname, base_match, base_regex)) { // sub_match 的第一个元素匹配整个字符串 // sub_match 的第二个元素匹配了第一个括号表达式 if (base_match.size() 2) { std::string base base_match[1].str(); std::cout sub-match[0]: base_match[0].str() std::endl; std::cout fname sub-match[1]: base std::endl; } } } #Variadic templates // C11 template typename First, typename... Args auto sum(const First first, const Args... args) - decltype(first) { const auto values {first, args...}; return std::accumulate(values.begin(), values.end(), First{0}); } // C17 template typename... Args auto sum(Args... args) { // Unary folding. return (... args); } template typename... Args bool logicalAnd(Args... args) { // Binary folding. return (true ... args); } bool b true; bool b2 b; logicalAnd(b, b2, true); // true #Inline namespaces namespace Program { namespace Version1 { int getVersion() { return 1; } bool isFirstVersion() { return true; } } inline namespace Version2 { int getVersion() { return 2; } } } int version {Program::getVersion()}; // Uses getVersion() from Version2 int oldVersion {Program::Version1::getVersion()}; // Uses getVersion() from Version1 bool firstVersion {Program::isFirstVersion()}; // Does not compile when Version2 is added # std::move temp struct Bar { int a 1; }; struct Foo { Bar getBar() { return bar; } Bar getBar() const { return Bar{}; } Bar getBar() { return std::move(Bar{}); } private: Bar bar; }; Foo foo; auto ref foo.getBar(); ref.a 10; # Nested namespaces namespace A::B::C { int i; } # __has_include #ifdef __has_include # if __has_include(optional) # include optional # define have_optional 1 # elif __has_include(experimental/optional) # include experimental/optional # define have_optional 1 # define experimental_optional # else # define have_optional 0 # endif #endif # std::invoke template typename Callable class Proxy { Callable c_; public: Proxy(Callable c) : c_{ std::move(c) } {} template typename... Args decltype(auto) operator()(Args... args) { // ... return std::invoke(c_, std::forwardArgs(args)...); } }; const auto add [](int x, int y) { return x y; }; Proxy p{ add }; p(1, 2); // 3 # std::clamp limitRange std::clamp(0, -1, 1, std::less{}); // 0 # std::reduce const std::arrayint, 3 a{ 1, 2, 3 }; std::reduce(std::cbegin(a), std::cend(a)); // 6 default binary operation is std::plus // Using a custom binary op: std::reduce(std::cbegin(a), std::cend(a), 1, std::multiplies{}); // 6 const std::arrayint, 3 b{ 1, 2, 3 }; const auto product_times_ten [](const auto a, const auto b) { return a * b * 10; }; std::transform_reduce(std::cbegin(a), std::cend(a), std::cbegin(b), 0, std::plus{}, product_times_ten); // 140 # std::inclusive_scan const std::arrayint, 3 a{ 1, 2, 3 }; std::inclusive_scan(std::cbegin(a), std::cend(a), std::ostream_iteratorint{ std::cout, }, std::plus{}); // 1 3 6 std::exclusive_scan(std::cbegin(a), std::cend(a), std::ostream_iteratorint{ std::cout, }, 0, std::plus{}); // 0 1 3 const auto times_ten [](const auto n) { return n * 10; }; std::transform_inclusive_scan(std::cbegin(a), std::cend(a), std::ostream_iteratorint{ std::cout, }, std::plus{}, times_ten); // 10 30 60 std::transform_exclusive_scan(std::cbegin(a), std::cend(a), std::ostream_iteratorint{ std::cout, }, 0, std::plus{}, times_ten); // 0 10 30 # GCD and LCM 公倍数 const int p 9; const int q 3; std::gcd(p, q); // 3 std::lcm(p, q); // 9 # std::not_fn # C20 Coroutines generatorint range(int start, int end) { while (start end) { co_yield start; start; } // Implicit co_return at the end of this function: // co_return; } for (int n : range(0, 10)) { std::cout n std::endl; } # concept template typename T concept unsigned_integral integralT !signed_integralT; template typename T concept callable requires (T f) { f(); }; // T is a constrained type template parameter. template typename T requires my_conceptT void f(T v); # [[likely]] and [[unlikely]] attributes switch (n) { case 1: // ... break; [[likely]] case 2: // n 2 is considered to be arbitrarily more // ... // likely than any other value of n break; } int random get_random_number_between_x_and_y(0, 3); if (random 0) [[likely]] { // body of if statement // ... } # constexpr virtual functions struct X1 { virtual int f() const 0; }; struct X2: public X1 { constexpr virtual int f() const { return 2; } }; # explicit(bool) struct foo { // Specify non-integral types (strings, floats, etc.) require explicit construction. template typename T explicit(!std::is_integral_vT) foo(T) {} }; # enum enum class rgba_color_channel { red, green, blue, alpha }; std::string_view to_string(rgba_color_channel my_channel) { switch (my_channel) { using enum rgba_color_channel; case red: return red; case green: return green; case blue: return blue; case alpha: return alpha; } } # constinit const char* g() { return dynamic initialization; } constexpr const char* f(bool p) { return p ? constant initializer : g(); } constinit const char* c f(true); // OK constinit const char* d f(false); // ERROR: g is not constexpr, so d cannot be evaluated at compile-time. # std::span std::spanconst int, 3{ c.cbegin(), c.cend() } # Bit operations std::popcount(0u); // 0 std::popcount(1u); // 1 std::popcount(0b11110000u); // 4 # Math constants std::numbers::pi; // 3.14159... std::numbers::e; // 2.71828.. # starts_with and ends_with on strings std::string str foobar; str.starts_with(foo); // true str.ends_with(baz); // false # std::midpoint std::midpoint(1, 3); // 2 # std::to_array int a[] {1, 2, 3}; std::to_array(a); // returns std::arrayint, 3C现代教程四_c float 转 string-CSDN博客创作不易小小的支持一下吧