局部变量作为非类型名称参数 [英] local variable as a non-typename argument
问题描述
为什么使用局部变量作为非类型参数是非法的?
Why is it illegal to use a local variable as a non-type argument?
例如,在下一个代码中 local_var
不能作为 X
的参数。
For example, in the next code local_var
cannot be argument to X
.
template<int& x> struct X {};
void f(int local_var)
{
X<local_var> x;
}
推荐答案
因为必须评估模板参数在编译时,编译器直到运行时才知道局部变量的地址(为了将引用绑定到对象,编译器需要知道该对象的地址)。
Because template arguments must be evaluated at compile time, and the compiler won't know the address of a local variable until run-time (in order to bind a reference to an object, the compiler needs to know the address of that object).
请注意,C ++ 11标准准确说明了在14.3.2 / 1段中可以提供哪些非类型模板参数:
Notice, that the C++11 Standard tells exactly what non-type template arguments can be provided in paragraph 14.3.2/1:
用于非类型,非模板 template-parameter 的 template-argument 应为:
A template-argument for a non-type, non-template template-parameter shall be one of:
-对于整数或枚举类型的非类型 template-parameter ,是类型的转换后的常量表达式
(5.19)模板参数;
— for a non-type template-parameter of integral or enumeration type, a converted constant expression (5.19) of the type of the template-parameter; or
-非类型模板参数的名称;或
— the name of a non-type template-parameter; or
-一个常数表达式(5.19),用于指定具有静态存储持续时间和
外部或内部链接或具有外部或内部链接的函数的对象的地址链接,包括函数模板
和函数 template-ids ,但不包括非静态类成员,表示为(忽略括号)为
& id-expression ,但&如果名称指向函数或数组,则可以省略;如果相应的模板参数为引用,则可以省略
;
— a constant expression (5.19) that designates the address of an object with static storage duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; or
—一个常量表达式,其值为空指针值(4.10);
— a constant expression that evaluates to a null pointer value (4.10); or
—一个常量表达式,其值为空成员指针值(4.11);或者
— a constant expression that evaluates to a null member pointer value (4.11); or
-指向成员的指针,如5.3.1所述;或
— a pointer to member expressed as described in 5.3.1; or
-类型为 std :: nullptr_t
的地址常量表达式。
— an address constant expression of type std::nullptr_t
.
如您所见,局部变量不在此列表中。
As you can see, local variables are not in this list.
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