C++

C++之static_cast

cplusplus.com中的解释：

“static_cast can perform conversions between pointers to related classes, not only upcasts (from pointer-to-derived to pointer-to-base), but also downcasts (from pointer-to-base to pointer-to-derived). No checks are performed during runtime to guarantee that the object being converted is in fact a full object of the destination type. Therefore, it is up to the programmer to ensure that the conversion is safe. On the other side, it does not incur the overhead of the type-safety checks of dynamic_cast. 

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classBase {};classDerived:publicBase {};
Base * a =newBase;
Derived * b =static_cast<Derived*>(a);

This would be valid code, although b would point to an incomplete object of the class and could lead to runtime errors if dereferenced.
Therefore, static_cast is able to perform with pointers to classes not only the conversions allowed implicitly, but also their opposite conversions.
static_cast is also able to perform all conversions allowed implicitly (not only those with pointers to classes), and is also able to perform the opposite of these. It can:

  • Convert from void* to any pointer type. In this case, it guarantees that if the void* value was obtained by converting from that same pointer type, the resulting pointer value is the same.
  • Convert integers, floating-point values and enum types to enum types.

Additionally, static_cast can also perform the following:

  • Explicitly call a single-argument constructor or a conversion operator.
  • Convert to rvalue references.
  • Convert enum class values into integers or floating-point values.
  • Convert any type to void, evaluating and discarding the value.

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从上面的内容来看，static_cast的功能还是很多的。记录重要的几点：

1. 相当于C语言的强制类型转换。如把int转成size_t：static_cast<size_t> i_value

2. 做基类的下行转换时，不安全，建议用dynamic_cast.

3. 用于基本数据类型之间的转换，如把int转换成char，把int转换成enum。

4. 把空指针转换成目标类型的空指针。

5. 把任何类型的表达式转换成void类型。

6. static_cast不能转换掉expression的const、volatile、或者__unaligned属性