C++核心準(zhǔn)則C.62:保證拷貝賦值對(duì)自我賦值安全

C.62: Make copy assignment safe for self-assignment
C.62:保證拷貝賦值對(duì)自我賦值安全

Reason（原因）

If x = x changes the value of x, people will be surprised and bad errors will occur (often including leaks).

如果x=x改變了x的值，人們會(huì)覺(jué)得很奇怪，同時(shí)也會(huì)發(fā)生很不好的錯(cuò)誤。（通常會(huì)包含泄露）

Example(示例)

The standard-library containers handle self-assignment elegantly and efficiently:

標(biāo)準(zhǔn)庫(kù)容器處理自我賦值的方式優(yōu)雅且高效：

std::vector v = {3, 1, 4, 1, 5, 9};
v = v;
// the value of v is still {3, 1, 4, 1, 5, 9}

Note（注意）

The default assignment generated from members that handle self-assignment correctly handles self-assignment.

產(chǎn)生于正確處理了自我賦值的成員的默認(rèn)的賦值操作會(huì)處理自我賦值問(wèn)題。

struct Bar {
 ? ?vector> v;
 ? ?map m;
 ? ?string s;
};

Bar b;
// ...
b = b; ? // correct and efficient

Note（注意）

You can handle self-assignment by explicitly testing for self-assignment, but often it is faster and more elegant to cope without such a test (e.g., using swap).

你可以通過(guò)明確地對(duì)自我賦值進(jìn)行檢查的方式防止自我賦值，但是通常不使用上述檢查的處理方式（例如使用swap）的方式更快，更優(yōu)雅。

class Foo {
 ? ?string s;
 ? ?int i;
public:
 ? ?Foo& operator=(const Foo& a);
 ? ?// ...
};

Foo& Foo::operator=(const Foo& a) ? // OK, but there is a cost
{
 ? ?if (this == &a) return *this;
 ? ?s = a.s;
 ? ?i = a.i;
 ? ?return *this;
}

This is obviously safe and apparently efficient. However, what if we do one self-assignment per million assignments? That's about a million redundant tests (but since the answer is essentially always the same, the computer's branch predictor will guess right essentially every time). Consider:

這種做法看起來(lái)安全并且高效。但是如果在一百萬(wàn)次賦值中只發(fā)生一次自我賦值的情況下會(huì)怎么樣呢？大概有一百萬(wàn)次多余的檢查（但是由于本質(zhì)上結(jié)果總是一樣的，計(jì)算機(jī)的分支預(yù)測(cè)會(huì)每次都猜對(duì)）?？紤]下面的代碼：

Foo& Foo::operator=(const Foo& a) ? // simpler, and probably much better
{
 ? ?s = a.s;
 ? ?i = a.i;
 ? ?return *this;
}

std::string is safe for self-assignment and so are int. All the cost is carried by the (rare) case of self-assignment.

std::string對(duì)自我賦值安全，int也是。所有的代價(jià)都來(lái)自（極少）發(fā)生的自我賦值。

Enforcement（實(shí)施建議）

(Simple) Assignment operators should not contain the pattern if (this == &a) return *this; ???

(簡(jiǎn)單)賦值運(yùn)算符不應(yīng)該包含以下的檢查：if (this == &a) return *this;

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