获取std :: complex< double>传递std :: is_floating_point测试 [英] Getting std::complex<double> to pass std::is_floating_point test

问题描述

我想要类型 double ， float ， complex< double> 和 complex< float> 以传递 static_assert 条件。我想到 static_assert（std :: is_floating< T> :: value，一些消息）会做的技巧，但复杂类型不通过这个测试下gcc-4.10）。

I want the types double, float, complex<double> and complex<float> to pass a static_assert condition. I figured static_assert(std::is_floating<T>::value, "some message") would do the trick, but the complex types do not pass this test (at least under gcc-4.10).

我需要添加什么谓词，以确保这四种类型（也许 long double 作为模板实例化，但没有别的？

What predicate would I add to make sure these four types (and perhaps long doubles as well) are allowed as template instantiations, but nothing else?

推荐答案

定义类型。 §20.10.2[meta.type.synop] / p1：

It is generally illegal to add specializations for standard library type trait classes, even for user-defined types. §20.10.2 [meta.type.synop]/p1:

程序的行为为任何$ b

The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified.

目前，唯一的类型trait类，用户如果专业化中至少有一个模板参数是用户定义的类型（§20.10.7.6[meta.trans。]），则允许为 std :: common_type 其他]，表57）。

Currently, the only type trait class that users are allowed to add specializations for is std::common_type, if at least one template parameter in the specialization is a user-defined type (§20.10.7.6 [meta.trans.other], Table 57).

您需要编写自己的trait，这并不困难：

You need to write your own trait, which isn't hard:

template<class T>
struct is_complex_or_floating_point : std::is_floating_point<T> { };

template<class T>
struct is_complex_or_floating_point<std::complex<T>> : std::is_floating_point<T> { };

演示 。

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