C++核心准则:C.164:避免隐式转换运算符

C.164: Avoid implicit conversion operators

C.164:避免隐式转换运算符

Reason(原因)

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

隐式转换可以很重要（例如，double转换为int），但经常会带来意外的结果(例如，String转换为C风格字符串)。

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.

优先采用显式命名转换，直到发现必须重视的需求。我们通过“必须重视的需求”来表达在应用领域中非常本质（例如整数到复数的转换）且经常遇到的原因。不要因为很小的便利而（通过转换运算符或者非显式构造函数）引入隐式转换。

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、具有潜在破坏的隐式转换可能在任何时候发生，而且难于发现。

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对象会在返回的指针被使用之前被销毁。

Enforcement(实施建议)

Flag all conversion operators.

提示所有的转换运算符。

原文链接：

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

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