引用参数的不同种类

1. 示例一

#include <iostream>
#include <string>struct Obj{Obj(){std::cout << "Ctor called.\n";}Obj(const Obj&){std::cout << "Copy ctor called.\n";    }~Obj(){std::cout << "Dtor called.\n";}
};void test1(const Obj& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}template<typename T>
void test2(const T& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}void test3(Obj& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}template<typename T>
void test4(T& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}void test5(Obj&& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}// 转发引用（万能引用）
template<typename T>
void test6(T&& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}int main () {Obj obj;const Obj obj2;test1(obj);test1(obj2);test1(Obj{});std::cout << std::string(30, '-') << std::endl;test2(obj);test2(obj2);test2(Obj{});std::cout << std::string(30, '-') << std::endl;test3(obj);//test3(obj2);test4(obj);test4(obj2);     // T&接受const Obj//test4(Obj{});  // T&不可能接受右值std::cout << std::string(30, '-') << std::endl;test5(Obj{});std::cout << std::string(30, '-') << std::endl;test6(obj);test6(obj2);test6(Obj{});std::cout << std::string(30, '-') << std::endl;
}

输出结果：

Ctor called.
Ctor called.
void test1(const Obj&)
void test1(const Obj&)
Ctor called.
void test1(const Obj&)
Dtor called.
------------------------------
void test2(const T&) [with T = Obj]
void test2(const T&) [with T = Obj]
Ctor called.
void test2(const T&) [with T = Obj]
Dtor called.
------------------------------
void test3(Obj&)
void test4(T&) [with T = Obj]
void test4(T&) [with T = const Obj]
------------------------------
Ctor called.
void test5(Obj&&)
Dtor called.
------------------------------
void test6(T&&) [with T = Obj&]
void test6(T&&) [with T = const Obj&]
Ctor called.
void test6(T&&) [with T = Obj]
Dtor called.
------------------------------
Dtor called.
Dtor called.

2. 示例二

#include <iostream>
#include <string>struct Obj{Obj(){std::cout << "Ctor called.\n";}Obj(const Obj&){std::cout << "Copy ctor called.\n";    }~Obj(){std::cout << "Dtor called.\n";}
};void test(const Obj& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}void test(Obj& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}void test(Obj&& obj){std::cout << __PRETTY_FUNCTION__ << std::endl;
}template<typename T>
void verifyValueCategory(T&& obj){if(std::is_lvalue_reference_v<T>){std::cout << "T is a lvalue reference\n";}else if(std::is_rvalue_reference_v<T>){std::cout << "T is a rvalue reference\n";}else {std::cout << "T is not a reference\n";}test(std::forward<T>(obj));
}template<typename T>
void verifyValueCategoryNoForward(T&& obj){if(std::is_lvalue_reference_v<T>){std::cout << "T is a lvalue reference\n";}else if(std::is_rvalue_reference_v<T>){std::cout << "T is a rvalue reference\n";}else {std::cout << "T is not a reference\n";}test(obj);
}int main () {Obj obj;const Obj obj2;std::cout << std::string(30, '-') << std::endl;verifyValueCategory(obj);     // void test(Obj&)verifyValueCategory(obj2);    // void test(const Obj&)verifyValueCategory(Obj{});   // void test(Obj&&)std::cout << std::string(30, '-') << std::endl;verifyValueCategoryNoForward(obj);verifyValueCategoryNoForward(obj2);verifyValueCategoryNoForward(Obj{});  // 这里会有问题，会调用左值引用的版本而不是右值引用的版本std::cout << std::string(30, '-') << std::endl;Obj&& r = Obj{};Obj&& r1 = Obj{};test(r);  // r是右值引用，但是r是变量因此属于左值，所以会调用void test(Obj&)的版本test(std::move(r));test(std::forward<decltype(r1)>(r1));std::cout << std::string(30, '-') << std::endl;
}

输出结果：

Ctor called.
Ctor called.
------------------------------
T is a lvalue reference
void test(Obj&)
T is a lvalue reference
void test(const Obj&)
Ctor called.
T is not a reference
void test(Obj&&)
Dtor called.
------------------------------
T is a lvalue reference
void test(Obj&)
T is a lvalue reference
void test(const Obj&)
Ctor called.
T is not a reference
void test(Obj&)
Dtor called.
------------------------------
Ctor called.
Ctor called.
void test(Obj&)
void test(Obj&&)
void test(Obj&&)
------------------------------
Dtor called.
Dtor called.
Dtor called.
Dtor called.