C++完美转发与引用折叠

C完美转发与引用折叠完美转发保持参数的值类别确保资源所有权正确传递。引用折叠规则决定模板参数推导后的引用类型。这两者是实现通用包装器的关键技术。引用折叠规则确定最终引用类型。#include#include#include#includetemplatevoid show_category(T arg) {if constexpr (std::is_lvalue_reference_v) {std::cout Lvalue reference (T T)\n;} else if constexpr (std::is_rvalue_reference_v) {std::cout Rvalue reference (T T)\n;} else {std::cout Not a reference\n;}}void reference_folding() {int x 42;std::cout Call with lvalue: ;show_category(x);std::cout Call with rvalue: ;show_category(100);}std::forward保持值类别进行转发。void process(int x) {std::cout Lvalue: x \n;}void process(int x) {std::cout Rvalue: x \n;}templatevoid forwarder(T arg) {process(std::forward(arg));}void forwarding_demo() {int x 10;std::cout Forwarding lvalue: ;forwarder(x);std::cout Forwarding rvalue: ;forwarder(20);}完美转发在工厂函数中的应用。templatestd::unique_ptr make_unique_fwd(Args... args) {return std::unique_ptr(new T(std::forward(args)...));}class Widget {std::string name_;int value_;public:Widget(const std::string name, int v): name_(name), value_(v) {std::cout Widget(const string)\n;}Widget(std::string name, int v): name_(std::move(name)), value_(v) {std::cout Widget(string)\n;}void show() const {std::cout name_ : value_ \n;}};void factory_demo() {std::string s test;auto w1 make_unique_fwd(s, 42);w1-show();auto w2 make_unique_fwd(std::string(temp), 100);w2-show();}可变参数模板与完美转发结合。templatevoid print_all(Args... args) {(std::cout ... std::forward(args)) \n;}void variadic_forward() {print_all(Hello, , World, , 42, , 3.14);}完美转发包装器。templatedecltype(auto) call_wrapper(Func func, Args... args) {std::cout Before call\n;if constexpr (std::is_void_v(args)...))) {func(std::forward(args)...);std::cout After call (void)\n;} else {auto result func(std::forward(args)...);std::cout After call (non-void)\n;return result;}}int add(int a, int b) {return a b;}void say_hello(const std::string name) {std::cout Hello, name !\n;}void wrapper_demo() {int result call_wrapper(add, 10, 20);std::cout Result: result \n;call_wrapper(say_hello, Alice);}引用折叠的四种情况。templatevoid fold_examples() {using lref T;using rref T;using T1 lref;using T2 lref;using T3 rref;using T4 rref;std::cout T - T: std::is_same_v \n;std::cout T - T: std::is_same_v \n;std::cout T - T: std::is_same_v \n;std::cout T - T: std::is_same_v \n;}void folding_rules() {fold_examples();}完美转发在emplace_back中的应用。class Expensive {std::string data_;public:Expensive(const std::string d) : data_(d) {std::cout Copy string\n;}Expensive(std::string d) : data_(std::move(d)) {std::cout Move string\n;}};void emplace_demo() {std::vector vec;std::string s Hello;std::cout push_back with copy:\n;vec.push_back(Expensive(s));std::cout emplace_back with move:\n;vec.emplace_back(std::move(s));}转发引用与auto。void auto_forward() {auto forwarded []() - int { return 42; }();std::cout Forwarded: forwarded \n;}完美转发是现代C泛型库开发的基础技术。