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）
代码语言：javascript复制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）
代码语言：javascript复制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）
代码语言：javascript复制template<typename T>
struct My_container {
public:
// ...
T& operator[](gsl::index i); // not unsigned
// ...
};Example（示例）
代码语言：javascript复制??? 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
