ES.40: Avoid complicated expressions
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）
代码语言：javascript复制// 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）
代码语言：javascript复制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。
- ???
链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es40-avoid-complicated-expressions
