括号在decltype（（c））中的意义？ [英] Significance of parentheses in decltype((c))?

问题描述

我正在阅读维基百科上关于 C ++ 11 的文章

有一个例子，我引用：

#include <vector>
int main() {  
    const std::vector<int> v(1);  
    auto a = v[0];        // a has type int  
    decltype(v[1]) b = 1; // b has type const int&, the return type of  
                          //   std::vector<int>::operator[](size_type) const  
    auto c = 0;           // c has type int  
    auto d = c;           // d has type int  
    decltype(c) e;        // e has type int, the type of the entity named by c  
    decltype((c)) f = c;  // f has type int&, because (c) is an lvalue  
    decltype(0) g;        // g has type int, because 0 is an rvalue  
}

：

    decltype(c) e;        // e has type int, the type of the entity named by c  
    decltype((c)) f = c;  // f has type int&, because (c) is an lvalue  

c 和（c）？为什么（c）表示左值？

推荐答案

• c 是变量的名称;

  （c）是一个表达式，在这种情况下是一个 lvalue 表达式，其值与变量 c

  (c) is an expression, in this case an lvalue expression, whose value is identical to the value of the variable c.

  c> decltype 。例如，考虑 decltype（1 + 2），这也是一个采用表达式的例子。它只是这样发生，你的例子是一个简单版本的表达式：一个只命名一个变量，并没有什么令人兴奋的。

  And the two are treated differently by decltype. Consider, for example, decltype(1+2), which is also an example taking an expression. It just so happens that your example is a simple version of an expression: one which merely names a single variable and does nothing exciting with it.

  这是其中一个差异，你通常只真正关心，如果你合理化的语言规范的微妙部分;

  It's one of those differences that you generally only really care about if you're rationalising about the subtle parts of the language specification; though, as you have identified, it has quite a significant practical effect in this case.

  请注意，这里没有操作符的用法。 。这一切都是从语法的布局中扣除的。

  Please note though that there is no operator usage here. It's all simply a deduction from the layout of the grammar.

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