c++ - 变量模板的部分特化

Question

我知道我可以部分特化类模板，我知道我不能部分特化函数模板。

variable templates呢？ ？我找不到关于它们是否可以部分专门化的文档。

Answer 1

是的，根据 [temp.arg.template]/2 :

Any partial specializations associated with the primary class template or primary variable template are considered when a specialization based on the template template-parameter is instantiated. ...

然后到 [temp.expl.spec]/7 :

... the placement of partial specialization declarations of class templates, variable templates, member class templates of non-template classes, static data member templates of non-template classes, member class templates of class templates, etc. ...

在[constraints.namespace.std]/3中也提到了:

The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

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更不用说所有主要编译器(Clang、GCC 和 MSVC)都没有问题:

template <int x, int y>
constexpr int foo = -1;

template <>
constexpr float foo<1, 0> = 1.0;

template <int x>
constexpr double foo<1, x> = 1.1;

int main()
{
    static_assert(foo<0, 0> == -1, "");
    static_assert(foo<0, 1> == -1, "");
    static_assert(foo<1, 0> == 1.0, "");
    static_assert(foo<1, 1> == 1.1, "");
}

https://godbolt.org/z/R1c1zo