c++ - 如何使用与成员初始值设定项列表一起使用的 static_assert

Question

我想使用 static_assert 对我的类的配置实现各种限制。早些时候，我只使用一个枚举，并且只允许一个需要所述枚举的构造函数来对我的类实现限制。如果我有类似下面的内容并且范围是从 0 到 4，这很好用，但是一旦我有 0 到 500 的范围，那么使用枚举就变得笨拙了。

一些类.h

class Some_Class {
    public:
        Some_Class(const unsigned int param);
    private:
        const unsigned int member_param;
};

Some_Class.cpp

Some_Class::Some_Class(const unsigned int param) : member_param(param) {
    static_assert(member_param < 500, "Param must be less than 500.");
};

main.cpp

Some_Class foo4(125); // OK
Some_Class foo5(500); // Should fail at compile time.

这是 GCC 在使用 C++14 编译时抛给我的:

1>  Some_Class.cpp: In constructor 'Some_Class::Some_Class(unsigned int)':
1>C:\some_path\Some_Class.cpp(3,2): error : non-constant condition for static assertion
1>    static_assert(member_param < 500, "Param must be less than 500.");
1>    ^
1>C:\some_path\Some_Class.cpp(3,2): error : use of 'this' in a constant expression

Answer 1

constexpr 中不能使用参数值。

你必须以某种方式提交编译时值:

• 为您的整个类(class)制作模板:

  template<unsigned int size>
  class Some_Class {
      static_assert(size < 500, "Size should be less than 500");
  public:
      constexpr unsigned int member_param = size;
  };
  

• 传递一个 integral_constant:

  template <unsigned int N>
  using uint_c = std::integral_constant<unsigned int, N>;
  
  class Some_Class {
  public:
      template<unsigned int size>
      Some_Class(uint_c<size>) : member_param(size)
      {
          static_assert(size < 500, "Size should be less than 500");
      }
  private:
      unsigned int member_param;
  };