C.46: By default, declare single-argument constructors explicit
C.46:默认状态下明确定义单参数构造函数
Reason（原因）
To avoid unintended conversions.
避免意外的转换。
Example, bad（反面示例）
class String {
public:
String(int); // BAD
// ...
};
String s = 10; // surprise: string of size 10
Exception(例外)
If you really want an implicit conversion from the constructor argument type to the class type, don't use explicit:
如果你确实需要一个从构造函数参数象类类型的隐式类型转换，不用适应explicit关键字。
class Complex {
public:
Complex(double d); // OK: we want a conversion from d to {d, 0}
// ...
};
Complex z = 10.7; // unsurprising conversion
See also: Discussion of implicit conversions
参见：关于隐式类型转换的讨论。
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Ro-conversion
Note（注意）
Copy and move constructors should not be made explicit because they do not perform conversions. Explicit copy/move constructors make passing and returning by value difficult.
拷贝和移动构造函数不应该定义为explict，应为那样就不会执行类型转换了。显示拷贝/移动构造函数使通过值传递参数和返回结果变得困难。
Enforcement（实施建议）
(Simple) Single-argument constructors should be declared explicit. Good single argument non-explicit constructors are rare in most code bases. Warn for all that are not on a "positive list".
(简单)唯一参数的构造函数应该被定义为explicit。定义良好的非explicit单参数构造函数在大多数代码中很少见。对于所有不在“正面清单”中的情况进行警告。
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c46-by-default-declare-single-argument-constructors-explicit
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