带有initializer_list的可选构造函数 [英] optional constructor with initializer_list
问题描述
这个特殊的构造函数使用初始化列表的目的是什么.有人可以举个例子说明什么时候会有用吗?
What is the purpose of this special constructor taking initializer list. Can someone give an example of when this will be useful?
template <class U, class... Args>
constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
以上内容与此有何不同?
How is the above different from this?
template <class... Args>
constexpr explicit optional(in_place_t, Args&&... args);
参考: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3793.html#optional.object.ctor
P.S.不确定使用c ++ 14还是c ++ 1z标签.我认为应该有c ++技术规范的标签
P.S. Not sure whether to use c++14 or c++1z tag. I think there should be tag for c++ techinical specification
推荐答案
使用两个单独的构造函数的原因是允许构造以initializer_list作为其构造函数参数的对象(可以选择在其后跟随任意参数列表) .假设您有一个类型如下的foo:
The reason for the two separate constructors is to allow construction of objects that take an initializer_list as their constructor argument (optionally followed by an arbitrary list of arguments). Say you have a type foo that looks like this:
struct foo
{
foo(std::initializer_list<int>) {}
};
在没有构造函数的情况下
In the absence of the constructor
template <class U, class... Args>
constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
您将无法将optional构造为
optional<foo> o(in_place, {1, 2, 3});
以上操作失败，因为 braced-init-list 没有类型，因此模板参数推导失败.您将不得不诉诸于此:
The above fails because a braced-init-list has no type, so template argument deduction fails. You'd have to resort to something like this:
auto il = {1, 2, 3};
optional<foo> o(in_place, il);
在构造optional对象时，具有接受initializer_list参数的构造函数可以使语法更加自然.
Having the constructor that accepts the initializer_list argument allows a more natural syntax when constructing the optional object.
这是一个最小示例，它展示了这两个构造函数的效用.
Here's a minimal example demonstrating the utility of the two constructors.
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