c++ - 仿函数和 initializer_list 的拷贝

Question

我对复制仿函数和/或初始化器时发生的情况感到有点困惑。在下面的代码中，我认为我会一直复制/移动对象，但不管它是否存在段错误。我似乎做错了什么，但还没有弄清楚，我的错误假设是什么。奇怪的是，在 cppreference.com 我找不到 initializer_list 的复制或移动构造函数，所以我想知道在这些情况下实际发生了什么。

#include <string>
#include <vector>
#include <functional>
#include <iostream>

std::initializer_list<std::function<std::string()>> getInitializer() {
  return {
    []() -> std::string {
      return "If";
    }
  };
}

int main() {
    std::function<int(std::string)> func;
    {
        auto init = getInitializer();

        func = [init](std::string text) -> int {
            std::vector<std::function<std::string()>> vec(init);

            for( auto& el : vec ) {
                std::cout << el();
            }
            std::cout << text << std::endl;
            return 5;
        };
    }

    return func(" you see this - the world is all right!");
}

Answer 1

我对 initializer_list 没有太多经验s，但标准似乎建议实现 initializer_list就好像它是一对指向数组的指针。名单在getInitializer具有自动生命周期，支持它的数组也是如此。您最终会返回一对指向不再存在的数组的指针。

标准的相关部分是 8.5.4 [decl.init.list] 第 5 项和第 6 项:

5.- An object of type std::initializer_list<E> is constructed from an initializer list as if the implementation allocated an array of N elements of type E, where N is the number of elements in the initializer list. Each element of that array is copy-initialized with the corresponding element of the initializer list, and the std::initializer_list<E> object is constructed to refer to that array. If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.

6.- The lifetime of the array is the same as that of the initializer_list object.

---

因此对于您的特定情况，实现大致等同于:

std::initializer_list<std::function<std::string()>> getInitializer() {
  std::function<std::string()> __a[1] = {
    []() -> std::string {
      return "If";
    }
  };
  return std::initializer_list<std::function<std::string()>>(__a, __a+1);
}