C++核心準(zhǔn)則C.148:使用dynamic_cast進行指針類型轉(zhuǎn)換時,將不能...

C.148: Use?dynamic_cast?to a pointer type when failure to find the required class is considered a valid alternative

C.148:使用dynamic_cast進行指針類型轉(zhuǎn)換時，將不能發(fā)現(xiàn)目標(biāo)類看作是有效的選項

Reason(原因)

The?dynamic_cast?conversion allows to test whether a pointer is pointing at a polymorphic object that has a given class in its hierarchy. Since failure to find the class merely returns a null value, it can be tested during run time. This allows writing code that can choose alternative paths depending on the results.

dynamic_cast轉(zhuǎn)換允許檢查是否指針指向一個在其繼承結(jié)構(gòu)中包含給定類的多態(tài)對象。由于轉(zhuǎn)換失敗的結(jié)果僅僅是返回一個空值，這個結(jié)果可以在執(zhí)行時檢查。這個特性允許根據(jù)結(jié)果選擇不同的路徑。

Contrast with?C.147, where failure is an error, and should not be used for conditional execution.

和C.147不同，那里的失敗是錯誤，而且不應(yīng)該被用于條件執(zhí)行。

Example（示例）

The example below describes the?add?function of a?Shape_owner?that takes ownership of constructed?Shape?objects. The objects are also sorted into views, according to their geometric attributes. In this example,?Shape?does not inherit from?Geometric_attributes. Only its subclasses do.

下面的例子描述的是Shape_owner的增加函數(shù)，它接受構(gòu)造出來的Shape對象的所有權(quán)。對象也會在根據(jù)它們的幾何屬性有序加入views容器。在這個例子中，圖形沒有從幾何屬性繼承。只有它的子類這么做了。

void add(Shape* const item)
{
  // Ownership is always taken
  owned_shapes.emplace_back(item);

  // Check the Geometric_attributes and add the shape to none/one/some/all of the views

  if (auto even = dynamic_cast(item))
  {
    view_of_evens.emplace_back(even);
  }

  if (auto trisym = dynamic_cast(item))
  {
    view_of_trisyms.emplace_back(trisym);
  }
}

Notes(注意)

A failure to find the required class will cause?dynamic_cast?to return a null value, and de-referencing a null-valued pointer will lead to undefined behavior. Therefore the result of the?dynamic_cast?should always be treated as if it may contain a null value, and tested.

尋找所需類的失敗會導(dǎo)致dynamic_cast返回一個空值，而解引用一個空指針會引起無定義的行為。因此應(yīng)該總是認(rèn)為dynamic_cast的結(jié)果可能為空并進行檢查。

Enforcement(實施建議)

• (Complex) Unless there is a null test on the result of a?dynamic_cast?of a pointer type, warn upon dereference of the pointer.

• (復(fù)雜)?如果在dynamic_cast執(zhí)行之后，沒有對結(jié)果指針進行空判斷，那么對使用這個指針的代碼報警。

原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c148-use-dynamic_cast-to-a-pointer-type-when-failure-to-find-the-required-class-is-considered-a-valid-alternative

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