c – GCC是否正确要求constexpr说明符用于此参考声明？

下面的代码不能在 GCC 5.3.0下编译,因为r的声明缺少constexpr说明符.

const int i = 1;
const int& r = i;
constexpr int j = r;

我相信拒绝是正确的.如何使用N4527工作草案证明这一点？

解决方法

首先,由于我们使用的是引用,因此不得违反[expr.const] /(2.9). (2.9.1)适用,但：

an id-expression that refers to a variable or data member of
reference type unless the reference has a preceding initialization and
either
— it is initialized with a constant expression

即使用r很好,只要初始化器 – i – 是一个常量表达式(如下所示).
还必须检查第3行中的l-t-r转换是否合法,即不得违反(2.7).但是,(2.7.1)适用：

an lvalue-to-rvalue conversion (4.1) unless it is applied to
— a
non-volatile glvalue of integral or enumeration type that refers to
a complete non-volatile const object with a preceding
initialization,initialized with a constant expression,or

……所以这很好,因为(g)左值是r,它指的是i – 这是一个带有常量表达式初始化器(1)的非易失性const对象.

我们推迟显示我实际上是一个常量表达式,一旦这样,我们需要证明r是一个常量表达式.
[expr.const] / 5与此有关：

A constant expression is either a glvalue core constant expression
whose value refers to an entity that is a permitted result of a
constant expression (as defined below),or a prvalue core constant
expression whose value is an object where,for that object and its
subobjects:

• each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,and
• if the object or subobject is of pointer type,it contains the address of an object with static storage duration,the address past
  the end of such an object (5.7),the address of a function,or a null
  pointer value.

An entity is a permitted result of a constant expression if it is an object with static storage duration that is
either not a temporary object or is a temporary object whose value satisfies the above constraints,or it is a
function.

因为在上面的上下文中,我是(g)左值,它必须是常量表达式的允许结果 – 因为它具有静态存储持续时间并且当然不是临时的.因此,我是一个不变的表达.

然而,r被视为第3行中的prvalue.由于我们已经确定r是核心常数表达式,我们只需要检查项目符号.不过,他们显然已经相遇了.

因此,代码在命名空间范围内格式良好.它不会在本地范围内,因为我不再是常量表达式的允许结果. Clang gives a comprehensive error message.

原文地址：https://www.jb51.cc/c/119442.html