C++1y/C++14 : Assignment to object outside its lifetime is not allowed in a constant expression?

Question

根据当前草案，以下 C++14/C++1y 程序是否格式错误？

#include <cstddef>

template<typename T, size_t n>
struct literal_array
{
    T data[n];
};

template<typename T, size_t n, size_t m>
constexpr literal_array<T, n+m> operator+(literal_array<T, n> a,
                                          literal_array<T, m> b)
{
    literal_array<T, n+m> x;

    for (size_t i = 0; i < n; i++)
        x.data[i] = a.data[i];

    for (size_t i = 0; i < m; i++)
        x.data[n+i] = b.data[i];

    return x;
}

int main()
{
    constexpr literal_array<int, 3> a = { 1, 2, 3 };
    constexpr literal_array<int, 2> b = { 4, 5 };

    constexpr auto c = a + b;
}

Clang trunk(在撰写本文时)给出:

error: constexpr variable 'c' must be initialized by a constant expression
        constexpr auto c = a + b;
                       ^   ~~~~~
assignment to object outside its lifetime is not allowed in a constant expression
                x.data[i] = a.data[i];
                          ^
in call to 'operator+({{1, 2, 3}}, {{4, 5}})'
        constexpr auto c = a + b;
                           ^

“在其生命周期之外分配给对象”是什么意思？ x 及其子对象的生命周期包含了函数，那么它是关于什么的呢？

Answer 1

程序是格式错误的，因为你没有初始化x，如果你把定义改成:

literal_array<T, n+m> x = {{0}};

clang 不再报错，它编译没有错误。另一种解决方案是创建 constexpr 构造函数。

我们可以在 draft standard 中找到它7.1.5 部分 constexpr 说明符 段落 3 说:

The definition of a constexpr function shall satisfy the following constraints:

并包括以下项目符号:

its function-body shall be = delete, = default, or a compound-statement that does not contain

其中包含此项目符号(强调我的):

a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.

然后我们有以下示例:

constexpr int uninit() {
  int a; // error: variable is uninitialized
  return a;
}

关于x 的生命周期的提示似乎不是建立在draft standard 中的。 .据我所知，正确的原因应该是 object is not initialized。

关于对象生命周期的标准草案中的相关引用将是 3.8 Object生命周期 段落 1 部分，它说:

The lifetime of an object is a runtime property of the object. An object is said to have non-trivial initialization if it is of a class or aggregate type and it or one of its members is initialized by a constructor other than a trivial default constructor. [ Note: initialization by a trivial copy/move constructor is non-trivial initialization. — end note ] The lifetime of an object of type T begins when:

• storage with the proper alignment and size for type T is obtained, and
• if the object has non-trivial initialization, its initialization is complete.

以防万一我遗漏了什么，我还使用 std::is_trivial 进行了检查:

std::cout <<  std::boolalpha << std::is_trivial<literal_array<int, 3>>::value << std::endl ;

和true中预期的结果，.

更新

我提交了 bug report为此，回复包括以下声明:

[...]The problem is that we don't yet implement the implied rule that such a function can't be invoked in a constant expression.