ES.40:避免复杂的表达式
Reason（原因）
Complicated expressions are error-prone.
复杂的表达式容易引发错误。
Example（示例）
// bad: assignment hidden in subexpression while ((c = getc()) != -1) // bad: two non-local variables assigned in sub-expressions while ((cin >> c1, cin >> c2), c1 == c2) // better, but possibly still too complicated for (char c1, c2; cin >> c1 >> c2 && c1 == c2;) // OK: if i and j are not aliased int x = ++i + ++j; // OK: if i != j and i != k v[i] = v[j] + v[k]; // bad: multiple assignments "hidden" in subexpressions x = a + (b = f()) + (c = g()) * 7; // bad: relies on commonly misunderstood precedence rules x = a & b + c * d && e ^ f == 7; // bad: undefined behavior x = x++ + x++ + ++x;Some of these expressions are unconditionally bad (e.g., they rely on undefined behavior). Others are simply so complicated and/or unusual that even good programmers could misunderstand them or overlook a problem when in a hurry.
上述代码中的有些表达式无论在什么情况下都是不好的（例如，它们依赖未定义的行为）。其他只是过于复杂，并且/或者不常见，从而使优秀的程序员也会错误理解或者匆忙中漏掉问题。
Note（注意）
C++17 tightens up the rules for the order of evaluation (left-to-right except right-to-left in assignments, and the order of evaluation of function arguments is unspecified; see ES.43), but that doesn't change the fact that complicated expressions are potentially confusing.
C++收紧了运算次序规则（除了赋值时从右到左之外都是从左到右，同时函数参数的运算次序是未定义的，参见ES.43），但是这也不会改变复杂的表达式可能引起混乱的事实。
Note（注意）
A programmer should know and use the basic rules for expressions.
程序员应该理解并运用关于表达式的基本准则。
Example（示例）
x = k * y + z; // OK auto t1 = k * y; // bad: unnecessarily verbose x = t1 + z; if (0 <= x && x < max) // OK auto t1 = 0 <= x; // bad: unnecessarily verbose auto t2 = x < max; if (t1 && t2) // ...Enforcement（实施建议）
Tricky. How complicated must an expression be to be considered complicated? Writing computations as statements with one operation each is also confusing. Things to consider:
难办。一个表达式到底复杂到什么程度算复杂？每个语句中只包含一个操作也会导致难以理解。需要考虑一下因素：
side effects: side effects on multiple non-local variables (for some definition of non-local) can be suspect, especially if the side effects are in separate subexpressions
副作用：可以怀疑多个非局部变量的副作用，特别是副作用存在于独立的子表达式中的情况。
writes to aliased variables
像变量的别名写入。
more than N operators (and what should N be?)
多余N个操作符（问题是N应该是几？）
reliance of subtle precedence rules
结果依赖于不易察觉的优先度规则。
uses undefined behavior (can we catch all undefined behavior?)
使用了未定义的行为（我们能找到未定义的行为么？）
implementation defined behavior?
又实现决定的行为。
???
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es40-avoid-complicated-expressions
觉得本文有帮助？欢迎点赞并分享给更多的人。
阅读更多更新文章，请关注微信公众号【面向对象思考】
