将指针作为模板参数：Comeau& MSVC编译，GCC失败 [英] Casting pointer as template argument: Comeau & MSVC compile, GCC fails

问题描述

请考虑以下代码：

template<int* a>
class base {};

int main()
{
    base<(int*)0> test;
    return 0;
}

Comeau和MSVC编译时没有问题（Comeau警告变量），而GCC在 base <（int *）0>上失败。测试; 行，说明

Both Comeau and MSVC compile this without issues (except for Comeau warning about an unused variable), while GCC fails on the base<(int*)0> test; line, stating

在函数`int main（）'中：
a转换为非整数或枚举类型的类型不能出现在常量表达式中

In function `int main()': a casts to a type other than an integral or enumeration type cannot appear in a constant-expression

模板参数1无效

它抱怨究竟是什么？谁是对的 - 这段代码应该编译吗？值得注意的是，我的GCC版本是非常 old（3.4.2），所以可能与它有关。感谢。

What exactly is it complaining about? And who's right -- should this code compile? It's worth noting that my GCC version is extremely old (3.4.2) so that may have something to do with it. Thanks.

推荐答案

从标准草案（着重号添加）：

14.1.3 A non-type template-parameter shall have one of the following (option-
  ally cv-qualified) types:
  ...
  --pointer to object, accepting an address constant  expression  desig-
    nating a named object with external linkage,
  ...

显然，使用空指针来实例化模板是不合法的，因为空指针不会指向具有外部链接的命名对象。

Apparently, it's not legal to instantiate a template with a null pointer, as a null pointer doesn't point to a "named object with external linkage".

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