C++核心准则​ES.71: 如果可以，使用范围for代替普通的for语句

ES.71: Prefer a range-for-statement to a for-statement when there is a choice

ES.71: 如果可以，使用范围for语句代替普通的for语句。

Reason（原因）

Readability. Error prevention. Efficiency.

可读性，防错和效率。

Example（示例）

for (gsl::index i = 0; i < v.size(); ++i)   // bad
        cout << v[i] << '\n';

for (auto p = v.begin(); p != v.end(); ++p)   // bad
    cout << *p << '\n';

for (auto& x : v)    // OK
    cout << x << '\n';

for (gsl::index i = 1; i < v.size(); ++i) // touches two elements: can't be a range-for
    cout << v[i] + v[i - 1] << '\n';

for (gsl::index i = 0; i < v.size(); ++i) // possible side effect: can't be a range-for
    cout << f(v, &v[i]) << '\n';

for (gsl::index i = 0; i < v.size(); ++i) { // body messes with loop variable: can't be a range-for
    if (i % 2 == 0)
        continue;   // skip even elements
    else
        cout << v[i] << '\n';
}

A human or a good static analyzer may determine that there really isn't a side effect on v in f(v, &v[i]) so that the loop can be rewritten.

"Messing with the loop variable" in the body of a loop is typically best avoided.

程序员或者好的静态分析软件或许可以判断f(v,&v[i])中的v实际上并不存在副作用，因此该循环可以被重写。通常情况下，最好避免在循环体中“乱用循环变量”。

Note（注意）

Don't use expensive copies of the loop variable of a range-for loop:

不要在循环体中进行代价高昂的循环变量拷贝。

for (string s : vs) // ...

This will copy each elements of vs into s. Better:

这会导致vs的每个元素都被拷贝。较好的做法是：

for (string& s : vs) // ...

Better still, if the loop variable isn't modified or copied:

如果循环变量不会被修改或拷贝，下面的做法更好。

for (const string& s : vs) // ...

Enforcement（实施建议）

Look at loops, if a traditional loop just looks at each element of a sequence, and there are no side effects on what it does with the elements, rewrite the loop to a ranged-for loop.

检查循环代码，如果一个传统的循环只是按照顺序读取每个元素，而且对元素的操作不存在副作用，使用范围for语句重写循环代码。

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es71-prefer-a-range-for-statement-to-a-for-statement-when-there-is-a-choice