book: add std::weak_ptr<T>::lock (changkun#216)

Author: Dw9

book/en-us/05-pointers.md

@@ -185,7 +185,8 @@ The solution to this problem is to use the weak reference pointer `std::weak_ptr
 
 In the above figure, only B is left in the last step, and B does not have any smart pointers to reference it, so this memory resource will also be released.
 
-`std::weak_ptr` has no `*` operator and `->` operator, so it can't operate on resources. Its only function is to check if `std::shared_ptr` exists, its `expired()` method can return `false` when the resource is not released, otherwise, it returns `true`.
+`std::weak_ptr` has no implemented `*` and `->` operators, therefore it cannot operate on resources. `std::weak_ptr` allows us to check if a `std::shared_ptr` exists or not. The `expired()` method of a `std::weak_ptr` returns `false` when the resource is not released; Otherwise, it returns `true`.
+Furthermore, it can also be used for the purpose of obtaining `std::shared_ptr`, which points to the original object. The `lock()` method returns a `std::shared_ptr` to the original object when the resource is not released, or `nullptr` otherwise.
 
 ## Conclusion
 

book/zh-cn/05-pointers.md

@@ -173,7 +173,7 @@ int main() {
 
 在上图中，最后一步只剩下 B，而 B 并没有任何智能指针引用它，因此这块内存资源也会被释放。
 
-`std::weak_ptr` 没有 `*` 运算符和 `->` 运算符，所以不能够对资源进行操作，它的唯一作用就是用于检查 `std::shared_ptr` 是否存在，其 `expired()` 方法能在资源未被释放时，会返回 `false`，否则返回 `true`。
+`std::weak_ptr` 没有 `*` 运算符和 `->` 运算符，所以不能够对资源进行操作，它可以用于检查 `std::shared_ptr` 是否存在，其 `expired()` 方法能在资源未被释放时，会返回 `false`，否则返回 `true`；除此之外，它也可以用于获取指向原始对象的 `std::shared_ptr` 指针，其 `lock()` 方法在原始对象未被释放时，返回一个指向原始对象的 `std::shared_ptr` 指针，进而访问原始对象的资源，否则返回`nullptr`。
 
 ## 总结