C++ Primer reading notes--universal reference and reference folding

Table of contents

1--Universal reference

1-1--The realization of universal reference

1-2--The difference between universal reference and rvalue reference

2--Reference folding

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1--Universal reference

1-1--The realization of universal reference

        A universal reference can pass any type of parameter to it , and it will automatically infer the parameter type ; the two implementations of the universal reference are as follows:

// 基于模板实现
template <typename T>
void f(T&& v){
}

// 基于auto实现
auto && v2 = v1;

1-2--The difference between universal reference and rvalue reference

        Different from using the above two methods to implement universal references (requiring type inference ) (C++ Primer also refers to universal references based on templates as rvalue references of a template type parameter ), in plain English, the type of rvalue references is deterministic , for example, the following are rvalue references with definite types :

int &&

char &&

std::string &&

        An rvalue reference itself is an lvalue , but it can only bind rvalues ;

        Universal references can bind lvalues , rvalues , and even const objects and non-const objects;

2--Reference folding

        In the template implementation of the universal reference above, when different types T are passed, reference folding often occurs ; when the passed parameter is a reference, the type T inferred by the template will be a reference to the reference , and then reference will occur fold;

In the template implementation of the universal reference, the rules of reference folding are as follows:

        ① When the passed parameter is an lvalue , the type T inferred by the template will be an lvalue reference ;

        ② When the passed parameter is a literal value , the type T inferred by the template will be the type of the literal value ;

        ③ In all cases (one exception, explained in point ④), the result of reference folding is an ordinary lvalue reference type;

        ④ But when the object of reference folding is an rvalue reference , the result of reference folding is an rvalue reference type;

// 基于模板实现
template <typename T>
void f(T&& v){
}

int x = 1;
f(x); // x是一个左值，则 T 的推断结果是左值引用: int &

f(1); // 1是一个字面值，则 T 的推断结果是字面值的类型: int

int&& y = 1; 
f(y); // y是一个右值引用，则 T 的推断结果是: int, T && 等价于 int &&;

// 对于一个给定的类型 X
X& &, X& &&, X&& & 都会折叠成X&, 
即int& &, int& &&, int&& & 都会折叠成(等价于)int&

类型X&& && 则会折叠成 X&&, 
即 int&& && 会折叠成(等价于)右值引用: int&&