C++核心準(zhǔn)則:C.164:避免隱式轉(zhuǎn)換運算符

C.164: Avoid implicit conversion operators

C.164:避免隱式轉(zhuǎn)換運算符

Reason(原因)

Implicit conversions can be essential (e.g.,?double?to?int) but often cause surprises (e.g.,?String?to C-style string).

隱式轉(zhuǎn)換可以很重要（例如，double轉(zhuǎn)換為int），但經(jīng)常會帶來意外的結(jié)果(例如，String轉(zhuǎn)換為C風(fēng)格字符串)。

Note(注意)

Prefer explicitly named conversions until a serious need is demonstrated. By "serious need" we mean a reason that is fundamental in the application domain (such as an integer to complex number conversion) and frequently needed. Do not introduce implicit conversions (through conversion operators or non-explicit constructors) just to gain a minor convenience.

優(yōu)先采用顯式命名轉(zhuǎn)換，直到發(fā)現(xiàn)必須重視的需求。我們通過“必須重視的需求”來表達(dá)在應(yīng)用領(lǐng)域中非常本質(zhì)（例如整數(shù)到復(fù)數(shù)的轉(zhuǎn)換）且經(jīng)常遇到的原因。不要因為很小的便利而（通過轉(zhuǎn)換運算符或者非顯式構(gòu)造函數(shù)）引入隱式轉(zhuǎn)換。

Example(示例)

struct S1 {
    string s;
    // ...
    operator char*() { return s.data(); }  // BAD, likely to cause surprises
};

struct S2 {
    string s;
    // ...
    explicit operator char*() { return s.data(); }
};

void f(S1 s1, S2 s2)
{
    char* x1 = s1;     // OK, but can cause surprises in many contexts
    char* x2 = s2;     // error (and that's usually a good thing)
    char* x3 = static_cast(s2); // we can be explicit (on your head be it)
}

The surprising and potentially damaging implicit conversion can occur in arbitrarily hard-to spot contexts, e.g.,

意外的、具有潛在破壞的隱式轉(zhuǎn)換可能在任何時候發(fā)生，而且難于發(fā)現(xiàn)。

S1 ff();

char* g()
{
    return ff();
}

The string returned by?ff()?is destroyed before the returned pointer into it can be used.

被ff()返回的string對象會在返回的指針被使用之前被銷毀。

Enforcement(實施建議)

Flag all conversion operators.

提示所有的轉(zhuǎn)換運算符。

原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c164-avoid-implicit-conversion-operators

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