initializer_list :: size()上的static_assert

Question

为什么st_ :: initializer_list <_E> :: size在static_assert中是不允许的,即使它在我的libstdc ++(v.4.6)中被声明为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() == Length, "Wrong number of dimensions");
    }
};

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

  return 0;
}

给出以下错误:

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

请注意,这适用于一个简单的示例:

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

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

  return 0;
}

Answer 1

"初始化列表"只是可怕的克拉格.

别:

#include <initializer_list>

template<typename T>
void Dont(std::initializer_list<T> list) { // Bad!
    static_assert(list.size() == 3, "Exactly three elements are required.");
}

void Test() { Dont({1,2,3}); }

做:

template<typename T, std::size_t N>
void Do(const T(&list)[N]) { // Good!
    static_assert(N == 3, "Exactly three elements are required.");
}

void Test() { Do({1,2,3}); }

Answer 2

编译器说init是问题,而不是init.size().

我猜可以从具有不同长度初始化器的不同位置调用构造函数.

(详细说明:你试图写一个static_assert取决于变量的运行时值init,即它有多少元素.static_assert在编译函数时必须可以计算.你的代码类似于这个无效的例子:)

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

Answer 3

从我与@Evgeny 的讨论中，我意识到这仅适用于（与gcc 4.8 c++11），并且也可以通过仅接受初始化列表（在main）中的兼容大小来进行大小检查。

（代码链接：http : //coliru.stacked-crooked.com/a/746e0ae99c518cd6）

#include<array>
template<class T, int Length>
class Point
{
  public:
    Point(std::array<T, Length> init)
    {
//not needed//      static_assert(init.size() == Length, "Wrong number of dimensions");
    }
};

int main()
{
  Point<int, 3> q({1,2,3}); //ok
//  Point<int, 3> q2({1,2,3,4}); //compile error (good!)
  Point<int, 3> q2({1,2}); // ok, compiles, same as {1,2,0}, feature or bug?
  return 0;
}