static_assert on initializer_list :: size（） [英] static_assert on initializer_list::size()

问题描述

为什么std :: initializer_list< _E> :: size在static_assert中不允许，即使它在我的libstdc ++（v.4.6）中被声明为一个constexpr？

例如，以下代码：

  template< class T，int Length> 
 class Point 
 {
 public：
 Point（std :: initializer_list< T> init）
 {
 static_assert（init.size长度，尺寸数错误）; 
} 
}; 
 
 int main（）
 {
 Point< int，3> q（{1,2,3}）; 
 
 return 0; 
} 
  

会出现以下错误：

  test.C：在构造函数'Point< T，Length> :: Point（std :: initializer_list< _Tp>）[with T = int，int Length = 3] ：
 test.C：60：26：从这里实例化
 test.C：54：7：错误：静态断言的非常量条件
 test.C：54：73：in constexpr扩展'init.std :: initializer_list< _E> :: size [with _E = int，std :: initializer_list< _E> :: size_type = long unsigned int]（）'
 test.C：54： 7：错误：'init'不是常量表达式
  

请注意，示例：

  class A 
 {
 public：
 constexpr int size（）{return 5; } 
}; 
 
 int main（）
 {
 A a; 
 static_assert（a.size（）== 4，oh no！）; 
 
 return 0; 
} 
  

解决方案

问题，而不是init.size（）。

我猜，构造函数可以从不同的地方用不同的长度初始化。

（详细说明：你试图写一个取决于变量的运行时值的 static_assert code> init ，即它有多少个元素。 static_assert 必须在编译函数时被赋值。类似于这个无效的例子：）

  void foo（int i）{static_assert（i == 42， ; } 
 int main（）{foo（42）; } //但是如果在另一个翻译单元中有一个调用者呢？ 
  

Why is std::initializer_list<_E>::size not allowable in a static_assert, even though it's declared as a constexpr in my libstdc++ (v. 4.6)?

For example, the following code:

template<class T, int Length>
class Point
{
  public:
    Point(std::initializer_list<T> init)
    {
      static_assert(init.size() == Length, "Wrong number of dimensions");
    }
};

int main()
{
  Point<int, 3> q({1,2,3});

  return 0;
}

gives the following error:

test.C: In constructor ‘Point<T, Length>::Point(std::initializer_list<_Tp>) [with T = int, int Length = 3]’:
test.C:60:26:   instantiated from here
test.C:54:7: error: non-constant condition for static assertion
test.C:54:73:   in constexpr expansion of ‘init.std::initializer_list<_E>::size [with _E = int, std::initializer_list<_E>::size_type = long unsigned int]()’
test.C:54:7: error: ‘init’ is not a constant expression

Note that this works just fine for a trivial example:

class A
{
  public:
    constexpr int size() { return 5; }
};

int main()
{
  A a;
  static_assert(a.size() == 4, "oh no!");

  return 0;
}

解决方案

The compiler says that init is the problem, not init.size().

I guess that the constructor could be called from different places with different length initializers.

(To elaborate: You're trying to write a static_assert that depends on the run-time value of the variable init, namely how many elements it has. static_asserts have to be evaluable at the time the function is compiled. Your code is analogous to this trivially invalid example:)

void foo(int i) { static_assert(i == 42, ""); }
int main() { foo(42); }  // but what if there's a caller in another translation unit?

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