C++ Primer 第 5 版第 16.5 章 Class- 模板特化

Question

我认为这本书的第 711 页有错误：

In § 16.2.3 (p. 684) we introduced the library remove_reference type. That template works through a series of specializations:

// original, most general template
template <class T> struct remove_reference {
    typedef T type;
};
// partial specializations that will be used fore lvalue and rvalue references
template <class T> struct remove_reference<T&> // lvalue references
    typedef T type;
};
template <class T> struct remove_reference<T&&> // rvalue references
    typedef T type;
};

...

int i;
// declyptype(42) is int, used the original template
remove_reference<decltype(42)>::type a;
// decltype(i) is int&, uses first(T&) partial specialization
remove_reference<decltype(i)>::type b;
// delctype(std::move(i)) is int&&, uses second (i.e., T&&) partial specialization
remove_reference<decltype(std::move(i))>::type c;

All three variables, a,b,and c, have type int.

我认为 decltype(i) 会产生一个普通的 int 而不是 int& 因此在 b 的情况下实际上使用了最通用的模板。对于普通变量类型 ^[1]，decltype 类型说明符生成普通类型，对于其他可用作 lvalue 的表达式，它将生成 [=26] =].

例子

#include <iostream>
#include <string>
#include <typeinfo>
using namespace std;

template <typename T> struct blubb {
    typedef T type;
    void print() { cout << "plain argument type\n"; }
};

template <typename T> struct blubb<T&> {
    typedef T type;
    void print() { cout << "lvalue reference type\n"; }
};

template <typename T> struct blubb<T&&> {
    typedef T type;
    void print() { cout << "rvalue reference type\n"; }
};

int main() {
    int i = 0;

    blubb<decltype(42)> plain;
    plain.print();
    blubb<decltype(i)> lvalue_ref; // actually not!
    lvalue_ref.print();
    
    int* pi = &i;
    blubb<decltype(*pi)> lvalue_ref2; // expression which can be on the left hand side
    lvalue_ref2.print();

    blubb<decltype(std::move(i))> rvalue_ref;
    rvalue_ref.print();

    return 0;
}

编译并运行

g++ -o types types.cpp -Wall -pedantic -g && ./types
plain argument type
plain argument type
lvalue reference type
rvalue reference type

请告诉我我是对还是错，并在适当的情况下进行解释。
谢谢

^[1] 可能正确的术语是 id-expression?

Answer 1

是的，对于未加括号的 id-expression，decltype 生成由 id-expression 命名的实体类型，然后 decltype(i) 生成类型 int.

If the argument is an unparenthesized id-expression or an unparenthesized class member access expression, then decltype yields the type of the entity named by this expression.

另一方面，decltype((i)) 产生类型 int&； (i) 被视为左值表达式。

If the argument is any other expression of type T, and
b) if the value category of expression is lvalue, then decltype yields T&;

Note that if the name of an object is parenthesized, it is treated as an ordinary lvalue expression, thus decltype(x) and decltype((x)) are often different types.

C++ Primer 第 5 版第 16.5 章 Class- 模板特化

C++ Primer 5th Edition Chapter 16.5 Class-Template Specializations

c++

decltype

errata

c++11