c++ - 仅由变量标识符组成的表达式类型

Question

cppreference说:

An identifier that names a variable, a function, specialization of a concept, (since C++20) or an enumerator can be used as an expression. The result of an expression consisting of just the identifier is the entity named by the identifier. [...] The type of the expression is determined as follows:

[...] Otherwise, the type of the expression is the same as the type of the entity named.

这不是一个错误吗？如果实体是引用，例如:

int &a = ...

那么a作为实体的类型是int &，而a作为表达式的类型只是int ，不是吗？表达式的类型是 never a reference :

[...] Each expression has some non-reference type

额外的问题:a 的类型(作为表达式)是 int 还是 int & 重要吗？如果是，它在哪些方面有所不同？

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注意:decltype 中使用了相同的措辞:

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.

但显然，decltype(a) 是 int &。

Answer 1

这是标准(草案)所说的:

[expr.type] Expressions / Type

If an expression initially has the type “reference to T”, the type is adjusted to T prior to any further analysis. ...

因此，很明显，表达式可以具有引用类型 - 最初，在类型分析调整之前。

"Each expression has some non-reference type" is not true?

取决于你如何理解它。一个表达式可以有一个非引用类型(最初)，但所有表达式都有一些非引用类型(调整后)用于类型分析。

[expr.prim.id.unqual] Expressions / Unqualified names

The result is the entity denoted by the identifier. ... If the entity is a template parameter object for a template parameter of type T ... [does not apply] ... Otherwise, the type of the expression is the type of the result. [Note: The type will be adjusted as described in 7.2.2 if it is cv-qualified or is a reference type. — end note] ... [Example:

void f() {
    float x, &r = x;
    [=] {
        decltype(x) y1;        // y1 has type float
        decltype((x)) y2 = y1; // y2 has type float const& because this lambda
                               // is not mutable and x is an lvalue
        decltype(r) r1 = y1;   // r1 has type float&
        decltype((r)) r2 = y2; // r2 has type float const&

    };
}

— end example]

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Decltype specifiers [dcl.type.decltype]

... if e is an unparenthesized id-expression or ..., decltype(e) is the type of the entity named by e.