左值与文字之间的比较是否会调用左值到右值转换？ [英] Does a Comparison Between an Lvalue and a Literal Invoke an Lvalue-to-Rvalue Conversion?

问题描述

我问了这个问题：常量变量的静态声明

并且显然归结为一个问题，为了比较的目的，浮点左值是否会转换为右值？

And apparently it comes down to the question does a floating point lvalue get converted to an rvalue for the purposes of comparison?

因此在此代码中，左值到

So in this code does an lvalue-to-rvalue conversion occur?

const float foo = 13.0F;
static_assert(foo > 0.0F, "foo must be greater than 0.");

推荐答案

是的，它已执行。基本上是因为 3.0> 1.2 是一个格式正确的表达式，除了操作数的prvalue外，不包含其他任何内容。

Yes, it is performed. Basically, it's all because 3.0 > 1.2 is a well-formed expression, that contains nothing but prvalues for operands.

首先， [expr] / 9 指出（强调我的意思）

First, [expr]/9 states (emphasis mine) that

每当glvalue表达式作为需要该操作数的prvalue的运算符
的操作数出现时，左值到右值
数组到指针或函数到指针的标准转换是应用
来将表达式转换为prvalue。

Whenever a glvalue expression appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue, array-to-pointer, or function-to-pointer standard conversions are applied to convert the expression to a prvalue.

因此，问题实际上归结为关系运算符是否期望用于操作数的prvalue ？答案也是肯定的。因为我们需要考虑 [expr.rel] / 1 ：

So the question really boils down to "Do the relational operators expect prvalues for operands"? And the answer to that is also yes. For we need to consider [expr.rel]/1:

relational-expression:
  shift-expression
  relational-expression < shift-expression
  relational-expression > shift-expression
  relational-expression <= shift-expression
  relational-expression >= shift-expression

操作数应具有算术，枚举或指针类型。
运算符< （小于），>（大于），< =（小于或等于），
和> =（大于或等于）全部产生false或true。

The operands shall have arithmetic, enumeration, or pointer type. The operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) all yield false or true. The type of the result is bool.

上面的语法产生是重要的一点。我们可以遵循它（我不会在这里完全完成操作）并将 shift-expression 减小为 primary-expression 。 主要表达式的结果之一是文字。在 [expr.prim.literal] ：

The above grammar production is the important bit. We can follow it (I won't do it entirely here) and reduce shift-expression to a primary-expression. And one of the productions of a primary-expression is a literal. For which it is said in [expr.prim.literal]:

文字是主要表达式。其类型取决于其形式。
字符串文字是左值；所有其他文字都是prvalues。

A literal is a primary expression. Its type depends on its form. A string literal is an lvalue; all other literals are prvalues.

因为大多数文字都是prvalues，我认为可以肯定地说关系运算符 expect 运算值的prvalue。

And because most literals are prvalues, I think it's safe to say the relational operators expect prvalues for operands.

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