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

ES.107:不要使用无符号数下标，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.

为了避免有符号数/无符号数混用带来的问题。有利实现更好的优化和错误检查。避免auto和int类型带来的陷阱。

Example, bad（反面示例）

vector<int> 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<int>::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<int> 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.

内置数组使用有符号数下标。标准库容器使用无符号数下标。因此不存在完美、完全兼容的解决方案（除非将来某一天标准库容器转而使用有符号数下标）。考虑到使用无符号数或者有符号数/无符号数混合可能带来的问题，较好的选择是赋予（有符号）整数足够大的空间，这一点可以通过使用gsl::index保证。

Example（示例）

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

Example（示例）

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

Alternatives（其他选项）

Alternatives for users

利用者角度的其他选项

• use algorithms
• 使用算法
• use range-for
• 使用范围for
• use iterators/pointers
• 使用指针和迭代器

Enforcement（实施建议）

• Very tricky as long as the standard-library containers get it wrong.
• 如果标准库容器出问题了，很难检出。
• (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.
• （为了避免误检出）如果一个操作数是sizeof或者container.size()而另一个操作数是ptrdiff_t，不要标记有符号数/无符号数混合的比较操作。

原文链接

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