为什么C ++不支持模板（非静态）成员变量？ [英] Why are template (non-static) member variables not supported in C++?

问题描述

虽然可以在C ++ 14中将静态成员变量模板化，但这将无法工作：

While static member variables can be templated in C++14 this wont work:

class SomeClass
{
  public:

  template<typename T>
  T var = {};
};

int main()
{
  SomeClass instance;
  instance.var<int> = 50;
  instance.var<double> = 0.1;
}

原因是什么，C ++不支持变量成员的模板

What are the reasons, that templates for variable members are not supported by the C++ standard since it should be possible in principle?

推荐答案

在原理上或实践上不可能 ，如其他答案所解释： sizeof（SomeClass）通常将无法计算，而 SomeClass 将不再计算具有任何可预测的或理智的身份，从而破坏了其存在的目的。

It cannot be possible in principle or in practice, as the other answers explain: sizeof(SomeClass) would be impossible to compute in general, and SomeClass would no longer have any predictable or sane identity, defeating the purpose of its existence.

如果只有少数几种类型希望从中选择，并且希望更改选择类型，也许您正在寻找 variant ？

If there are only a select few types you wish to choose from, and you wish to change the "selected" type at runtime, perhaps a variant is what you're looking for?

#include <variant>

class SomeClass
{
public:
   std::variant<int, double> var = {};
};

int main()
{
   SomeClass instance;
   instance.var = 50;
   instance.var = 0.1;
}

（此需要C ++ 17 ，但已经有很多年了等效的Boost。）

(This requires C++17, but a Boost equivalent has been available for many, many years.)

它之所以有效，是因为 var 会和存储 一个 int所需的大小一样大或 double （加上一些内务处理），无论您的变体在任何给定时间处于哪种模式，此大小都是固定的。

It works because var will be as big as it needs to to store either an int or a double (plus some housekeeping), and this size is fixed no matter which "mode" your variant is in at any given time.

如果要接受任何类型，则可以使用 std :: any ，就像毒品的变体一样。开销比较重，但是如果您的需求真的很放松，那么就可以完成工作。

If you want to accept any type, you could use std::any, which is like a variant on drugs. The overhead is a little heavier, but if your requirements are really so relaxed then this can do the job.

但是如果您想要多个变量，有多个变量。

But if you want multiple variables, have multiple variables.

这篇关于为什么C ++不支持模板（非静态）成员变量？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！