是否所有constexpr变量都隐式内联? [英] Are all constexpr variable implicitly inline?

问题描述

我在玩自动模板参数，而令我惊讶的是这段代码没有编译:

I was playing around with auto template parameters and I was surprised that this code didn't compiled:

constexpr auto bar = 2;

template<auto& T>
struct Foo {
    auto operator()() const { return T; }
};

int main() {
    Foo<bar> b;
    b();
}

Visual Studio 15.7(预览版4)吐出这些错误:

Visual Studio 15.7 (preview 4) spit out these errors:

error C2970: 'Foo': template parameter 'T': 'bar': an expression involving objects with internal linkage cannot be used as a non-type argument
 note: see declaration of 'Foo'
 note: see declaration of 'bar'
error C2440: 'specialization': cannot convert from 'int' to 'int &'
 note: see reference to class template instantiation 'Foo<0>' being compiled
error C2973: 'Foo': invalid template argument 'int'
 note: see declaration of 'Foo'

然后，添加inline后，错误消失了！

Then, after adding inline, the error got away!

constexpr inline auto bar = 2;

我认为constexpr变量是隐式inline.另外，这如何影响变量bar的链接?

I thought constexpr variables were implicitly inline. Also, how does that affect the linkage of my variable bar?

推荐答案

所有constexpr变量是否隐式内联?

Are all constexpr variable implicitly inline?

不.仅constexpr函数和constexpr静态数据成员是隐式内联的( [dcl. constexpr]/1 ).

No. Only constexpr functions and constexpr static data members are implicitly inline ([dcl.constexpr]/1).

还有，这如何影响我的变量栏的链接?

Also, how does that affect the linkage of my variable bar?

constexpr变量为const( [dcl.constexpr ]/9 ).未显式声明为extern的非内联const变量具有内部链接( [basic.link]/3 ).

A constexpr variable is const ([dcl.constexpr]/9). A non-inline const variable that is not explicitly declared extern has internal linkage ([basic.link]/3).

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