ES.107: Don't use?unsigned?for subscripts, prefer?gsl::index

ES.107:不要使用無符號數下標，使用gsl::index更好

Reason(原因)

To avoid signed/unsigned confusion. To enable better optimization. To enable better error detection. To avoid the pitfalls with?auto?and?int.

為了避免有符號數/無符號數混用帶來的問題。有利實現更好的優化和錯誤檢查。避免auto和int類型帶來的陷阱。

Example, bad(反面示例)

vector vec = /*...*/;
for (int i = 0; i < vec.size(); i += 2) // may not be big enough
cout << vec[i] << '\n';
for (unsigned i = 0; i < vec.size(); i += 2) // risk wraparound
cout << vec[i] << '\n';
for (auto i = 0; i < vec.size(); i += 2) // may not be big enough
cout << vec[i] << '\n';
for (vector::size_type i = 0; i < vec.size(); i += 2) // verbose
cout << vec[i] << '\n';
for (auto i = vec.size()-1; i >= 0; i -= 2) // bug
cout << vec[i] << '\n';
for (int i = vec.size()-1; i >= 0; i -= 2) // may not be big enough
cout << vec[i] << '\n';

Example, good(范例)

vector vec = /*...*/;
for (gsl::index i = 0; i < vec.size(); i += 2) // ok
cout << vec[i] << '\n';
for (gsl::index i = vec.size()-1; i >= 0; i -= 2) // ok
cout << vec[i] << '\n';

Note(注意)

The built-in array uses signed subscripts. The standard-library containers use unsigned subscripts. Thus, no perfect and fully compatible solution is possible (unless and until the standard-library containers change to use signed subscripts someday in the future). Given the known problems with unsigned and signed/unsigned mixtures, better stick to (signed) integers of a sufficient size, which is guaranteed by?gsl::index.

內置數組使用有符號數下標。標準庫容器使用無符號數下標。因此不存在完美、完全兼容的解決方案(除非將來某一天標準庫容器轉而使用有符號數下標)。考慮到使用無符號數或者有符號數/無符號數混合可能帶來的問題，較好的選擇是賦予(有符號)整數足夠大的空間，這一點可以通過使用gsl::index保證。

Example(示例)

template
struct My_container {
public:
// ...
T& operator[](gsl::index i); // not unsigned
// ...
};

Example(示例)

??? demonstrate improved code generation and potential for error detection ???

Alternatives(其他選項)

Alternatives for users

利用者角度的其他選項

• use algorithms

• 使用算法

• use range-for

• 使用范圍for

• use iterators/pointers

• 使用指針和迭代器

Enforcement(實施建議)

• Very tricky as long as the standard-library containers get it wrong.

• 如果標準庫容器出問題了，很難檢出。

• (To avoid noise) Do not flag on a mixed signed/unsigned comparison where one of the arguments is?sizeof?or a call to container?.size()?and the other is?ptrdiff_t.

• (為了避免誤檢出)如果一個操作數是sizeof或者container.size()而另一個操作數是ptrdiff_t，不要標記有符號數/無符號數混合的比較操作。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es107-dont-use-unsigned-for-subscripts-prefer-gslindex

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