问题描述
此代码
int clash;
struct Foo {
decltype(clash) clash;
};
在 clang 上静默编译,但在 gcc 上编译失败并出现错误
compiles silently on clang, but fails to compile on gcc giving the errors
错误:'int Foo::clash'的声明[-fpermissive]
error: declaration of 'int Foo::clash' [-fpermissive]
错误:将 'clash' 的含义从 'int crash' [-fpermissive] 更改为
error: changes meaning of 'clash' from 'int clash' [-fpermissive]
似乎需要 2 个因素才能出现错误:
It seems that 2 ingredients are required for the error to arise:
阴影必须由类成员完成(如果是函数的局部作用域,没问题).
The shadowing must be done by a class member (no problem if it's a function's local scope).
decltype([shadowed name]) 必须在声明[shadowing name]之前的shadowing作用域中使用.
decltype([shadowed name]) must be used in the shadowing scope before the declaration of [shadowing name].
我的问题是双重的:
- gcc 拒绝此代码是否合理?
- 标准中哪里这么说的?
推荐答案
gcc
是正确的程序格式错误,尽管这种特殊的违规不需要诊断所以 clang
代码>不必提供.
gcc
is correct the program is ill-formed, although this particular violation does not require a diagnostic so clang
does not have to provide one.
如果我们看一下 C++11 标准(最接近的草案是 N3337) 部分 3.3.7
Class scope 它说:
If we look at the C++11 standard(The closest draft would be N3337) section 3.3.7
Class scope it says:
在类 S 中使用的名称 N 应在其上下文以及在 S 的完整范围内重新评估时.否违反此规则需要进行诊断.
A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.
然后下一条规则说:
如果重新排序类中的成员声明会产生一个替代的有效(1) 和 (2) 下的程序,程序格式错误,没有诊断需要.
If reordering member declarations in a class yields an alternate valid program under (1) and (2), the program is ill-formed, no diagnostic is required.
我们希望防止重新排序类中的声明给出不同程序的情况是有道理的.很好奇这两条规则是否多余.
It makes sense we would want to prevent situations where reordering the declarations in a class give a different program. It is curious whether these two rules are redundant or not.
该部分还提供了以下示例:
The section also provides the following example:
enum { i = 1 };
class X {
char v[i]; // error: i refers to ::i
// but when reevaluated is X::i
int f() { return sizeof(c); } // OK: X::c
char c;
enum { i = 2 };
};
如果我们用 gcc
(现场观看),我们得到的错误与您的代码产生的错误几乎相同:
and if we try this example with gcc
(see it live), we get an almost identical error to one your code produces:
error: declaration of 'i' [-fpermissive]
enum { i = 2 };
^
error: changes meaning of 'i' from '<anonymous enum> i' [-fpermissive]
enum { i = 1 };
这篇关于遮蔽外部名称的 decltype 和类成员名称之间的交互的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持跟版网!