问题描述

这个 lambda 递归是否有效?

#include #include int main() {std::functiong = [&g](int k) {返回 (k ? k * g(k-1) : 1);};std::cout <<克(10)；//3628800}

它似乎可以编译和运行，但我对关闭 g 感到紧张与我初始化它的语句相同.在 1-10 的范围内严格效度...?

在您通过引用捕获 g 时，它已被声明，因此该名称可供使用:<块引用>

3.3.2/1 名称的声明点紧跟在其完整声明符(第 8 条)之后和初始化符之前

您可以在初始化之前以有限的方式使用对象 - 基本上，任何不依赖于值的东西都可以:

3.8/6 在对象的生命周期开始之前但在对象将占用的存储空间之后已分配 [...] 任何引用原始对象的泛左值都可以使用，但仅限于有限的方法.[...] 使用不依赖于其值的泛左值的属性是明确定义的.

所以根据我的理解，你正在做的事情是明确定义的.

(虽然是超迂腐，我不认为它是在分配自动对象的存储时指定的，并且 8.3.2/5 说应初始化引用以引用有效对象"而没有定义有效"，所以有理由认为它没有明确定义).

Is this lambda recursion valid?

#include <functional>
#include <iostream>

int main() {
   std::function<int(int)> g = [&g](int k) {
       return (k ? k * g(k-1) : 1);
   };

   std::cout << g(10); // 3628800
}

It appears to compile and run ok, but I'm nervous about closing over g in the same statement that I initialise it. Strict validity on a scale of 1-10...?

At the point at which you capture g by reference, it has been declared, so the name is available for use:

3.3.2/1 The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer

You are allowed to use objects in limited ways before they are initialised - basically, anything that doesn't depend on the value is OK:

3.8/6 before the lifetime of an object has started but after the storage which the object will occupy has been allocated [...] any glvalue that refers to the original object may be used but only in limited ways. [...] using the properties of the glvalue that do not depend on its value is well-defined.

So by my understanding, what you are doing is well-defined.

(Although, being ultrapedantic, I don't think it's specified when the storage for an automatic object is allocated, and 8.3.2/5 says that "a reference shall be initialized to refer to a valid object" without defining "valid", so there's scope to argue that it's not well-defined).

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