C++核心准则ES.49:如果必须进行类型转换，使用命名转换

ES.49: If you must use a cast, use a named cast

ES.49:如果必须进行类型转换，使用命名转换

Reason（原因）

Readability. Error avoidance. Named casts are more specific than a C-style or functional cast, allowing the compiler to catch some errors.

可读性。避免错误。命名转换比C风格转换或函数形式转换更明确，允许编译器捕捉更多错误。

The named casts are:

命名转换包括：

• static_cast
• 静态转换
• const_cast
• 常量转换
• reinterpret_cast
• 重新解释转换
• dynamic_cast
• 动态转换
• std::move // move(x) is an rvalue reference to x
• 移动//move(x)是x的右值引用
• std::forward // forward<T>(x) is an rvalue or an lvalue reference to x depending on T
• 值性转递//根据T的类型，forward<T>(x)是左值或右值引用
• gsl::narrow_cast // narrow_cast<T>(x) is static_cast<T>(x)
• 窄化转换//narrow_cast<T>(x)就是static_cast<T>(x)
• gsl::narrow // narrow<T>(x) is static_cast<T>(x) if static_cast<T>(x) == x or it throws narrowing_error
• 窄化转换（抛出异常）//如果static_cast<T>(x) == x，narrow<T>(x) 就是 static_cast<T>(x)，否则抛出异常

Example（示例）

class B { /* ... */ };
class D { /* ... */ };

template<typename D> D* upcast(B* pb)
{
    D* pd0 = pb;                        // error: no implicit conversion from B* to D*
    D* pd1 = (D*)pb;                    // legal, but what is done?
    D* pd2 = static_cast<D*>(pb);       // error: D is not derived from B
    D* pd3 = reinterpret_cast<D*>(pb);  // OK: on your head be it!
    D* pd4 = dynamic_cast<D*>(pb);      // OK: return nullptr
    // ...
}

The example was synthesized from real-world bugs where D used to be derived from B, but someone refactored the hierarchy. The C-style cast is dangerous because it can do any kind of conversion, depriving us of any protection from mistakes (now or in the future).

示例是从实际代码中收集的的错误集合，这段代码的前提是D过去继承于B，但有人重构了继承关系。C风格转换的危险性来自它可以是任何类型的转换，这抹杀了任何防错保护的可能性（无论是现在还是未来）。

Note（注意）

When converting between types with no information loss (e.g. from float to double or from int32 to int64), brace initialization may be used instead.

如果希望在类型之间进行无损转换（例如从float到double，或者从int32到int64），可以考虑转而使用大括号初始化。

double d {some_float};
int64_t i {some_int32};

This makes it clear that the type conversion was intended and also prevents conversions between types that might result in loss of precision. (It is a compilation error to try to initialize a float from a double in this fashion, for example.)

这种方式一方面明确了类型转换的意图，另一方面可以防止转换时损失精度。（例如，在如代码所示的情况下，如果使用double值初始化float变量，会发生编译错误）

Note（注意）

reinterpret_cast can be essential, but the essential uses (e.g., turning a machine address into pointer) are not type safe:

reinterpret_cast是必不可少的，但是这种必要的用法（例如，将机器地址转换为指针）不是类型安全的。

auto p = reinterpret_cast<Device_register>(0x800);  // inherently dangerous

Enforcement（实施建议）

• Flag C-style and functional casts.
• 对C风格和函数形式转换进行提醒
• The type profile bans reinterpret_cast.
• 类型规则群组禁止reinterpret_cast.
• The type profile warns when using static_cast between arithmetic types.
• 类型规则群组对在算数类型之间进行转换时使用static_cast的情况进行警告。

译者注：

C风格转换：b = int(a);

函数形式转换：b=int(a);

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es49-if-you-must-use-a-cast-use-a-named-cast