C++核心準(zhǔn)則ES.107:不要使用無(wú)符號(hào)數(shù)下標(biāo),使用gsl::index更好?

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

ES.107:不要使用無(wú)符號(hào)數(shù)下標(biāo)，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(hào)數(shù)/無(wú)符號(hào)數(shù)混用帶來(lái)的問(wèn)題。有利實(shí)現(xiàn)更好的優(yōu)化和錯(cuò)誤檢查。避免auto和int類型帶來(lái)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(nèi)置數(shù)組使用有符號(hào)數(shù)下標(biāo)。標(biāo)準(zhǔn)庫(kù)容器使用無(wú)符號(hào)數(shù)下標(biāo)。因此不存在完美、完全兼容的解決方案（除非將來(lái)某一天標(biāo)準(zhǔn)庫(kù)容器轉(zhuǎn)而使用有符號(hào)數(shù)下標(biāo)）?？紤]到使用無(wú)符號(hào)數(shù)或者有符號(hào)數(shù)/無(wú)符號(hào)數(shù)混合可能帶來(lái)的問(wèn)題，較好的選擇是賦予（有符號(hào)）整數(shù)足夠大的空間，這一點(diǎn)可以通過(guò)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(xiàng)）

Alternatives for users

利用者角度的其他選項(xiàng)

• 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(shí)施建議）

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

• 如果標(biāo)準(zhǔn)庫(kù)容器出問(wèn)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(gè)操作數(shù)是sizeof或者container.size()而另一個(gè)操作數(shù)是ptrdiff_t，不要標(biāo)記有符號(hào)數(shù)/無(wú)符號(hào)數(shù)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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