C++核心準則ES.40:避免復雜的表達式

ES.40: Avoid complicated expressions

ES.40:避免復雜的表達式

Reason（原因）

Complicated expressions are error-prone.

復雜的表達式容易引發(fā)錯誤。

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.

上述代碼中的有些表達式無論在什么情況下都是不好的（例如，它們依賴未定義的行為）。其他只是過于復雜，并且/或者不常見，從而使優(yōu)秀的程序員也會錯誤理解或者匆忙中漏掉問題。

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++收緊了運算次序規(guī)則（除了賦值時從右到左之外都是從左到右，同時函數(shù)參數(shù)的運算次序是未定義的，參見ES.43），但是這也不會改變復雜的表達式可能引起混亂的事實。

Note（注意）

A programmer should know and use the basic rules for expressions.

程序員應該理解并運用關(guān)于表達式的基本準則。

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（實施建議）

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:

難辦。一個表達式到底復雜到什么程度算復雜？每個語句中只包含一個操作也會導致難以理解。需要考慮一下因素：

• 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：可以懷疑多個非局部變量的副作用，特別是副作用存在于獨立的子表達式中的情況。

• writes to aliased variables

• 像變量的別名寫入。

• more than N operators (and what should N be?)

• 多余N個操作符（問題是N應該是幾？）

• reliance of subtle precedence rules

• 結(jié)果依賴于不易察覺的優(yōu)先度規(guī)則。

• uses undefined behavior (can we catch all undefined behavior?)

• 使用了未定義的行為（我們能找到未定義的行為么？）

• implementation defined behavior?

• 又實現(xiàn)決定的行為。

• ???

  
  

原文鏈接

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

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