C++核心准则ES.40:避免复杂的表达式

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）

// 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。
• ???

链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es40-avoid-complicated-expressions