`reinterpret_cast`

C++中reinterpret_cast的目的是是实现：alias to an existing object，这在C++\Language-reference\Basic-concept\Data-model\Object\Object.md#Aliase to an existing object 中进行了详细介绍。

pointer、reference的conversion都使用reinterpret_cast 。

参见:

pointer: C-family-language\C-and-C++\Pointer-array-alias\Pointer
reference: C++\Language-reference\Reference

cppreference reinterpret_cast conversion

Converts between types by reinterpreting the underlying bit pattern.

NOTE: 上面这段话所描述的就是alias，这在C++\Language-reference\Basic-concept\Data-model\Object\Object.md#Aliase to an existing object 中进行了详细介绍。

Explanation

Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility.

NOTE: 上面这段话的意思是：reinterpret_cast是需要保证CV的，结合下面的例子来进行说明：
#include <iostream>

void func(const void* input)
{
  const int* i = reinterpret_cast<const int*>(input); // 保持CV
  std::cout << *i << std::endl;
}
int main()
{
  const int i = 0;
  func(&i);
}
// g++ test.cpp
函数func的入参是const的，reinterpret_cast的时候，是需要保持CV的，否则会编译报错，如下是错误示例：
#include <iostream>

void func(const void* input)
{
  int* i = reinterpret_cast<int*>(input); // 丢失CV
  std::cout << *i << std::endl;
}
int main()
{
  const int i = 0;
  func(&i);
}
上述程序报错如下：
test.cpp: 在函数‘void func(const void*)’中:
test.cpp:5:39: 错误：从类型‘const void*’到类型‘int*’的 reinterpret_cast 丢失了限定符
  int* i = reinterpret_cast<int*>(input);
上述程序，可以通过const_cast来 drop CV，在“reinterpret_cast VS C-style cast”章节对它进行了详细描述。

Integral type and pointer

NOTE: 关于integral type 和 pointer之间的转换，在C-family-language\C-and-C++\Pointer-array-alias\Pointer-and-integer.md中对此进行了详细的说明。

1)

NOTE: 原文描述的规则没有理解

2) pointer to integral type

A pointer can be converted to any integral type large enough to hold all values of its type (e.g. to std::uintptr_t)

A pointer converted to an integer of sufficient size and back to the same pointer type is guaranteed to have its original value, otherwise the resulting pointer cannot be dereferenced safely (the round-trip conversion in the opposite direction is not guaranteed; the same pointer may have multiple integer representations)

NOTE: 为了保证“sufficient size”，最好使用C99中引入的intptr_t、uintptr_t ，关于此，在cmu.edu INT36-C. Converting a pointer to integer or integer to pointer中进行了探讨，在C-and-C++\Pointer-array-alias\Pointer-and-integer.md中收录了这篇文章。

关于intptr_t、uintptr_t ，可以参见：

stackoverflow What is the use of intptr_t?

stackoverflow Why / when to use intptr_t for type-casting in C?

The null pointer constant NULL or integer zero is not guaranteed to yield the null pointer value of the target type; static_cast or implicit conversion should be used for this purpose.

NOTE: 原文中的，这一段话没有理解。

3) integral or enumeration type to pointer

4) `std::nullptr_t` to integral type

Type aliasing

5) type aliasing through pointer

Examples

Object#Object representation and value representation

#include <cassert>
struct S
{
    char c;  // 1 byte value
             // 3 bytes padding (assuming alignof(float) == 4)
    float f; // 4 bytes value (assuming sizeof(float) == 4)
    bool operator==(const S& arg) const
    { // value-based equality
        return c == arg.c && f == arg.f;
    }
};

void f()
{
    assert(sizeof(S) == 8);
    S s1 = { 'a', 3.14 };
    S s2 = s1;
    reinterpret_cast<unsigned char*>(&s1)[2] = 'b'; // change 2nd byte of padding
    assert(s1 == s2); // value did not change
}
int main()
{
    f();
}
// g++ test.cpp

stackoverflow How to print (using cout) a number in binary form? # A

#include <iostream>
#include <bitset>
#include <climits>

template<typename T>
void show_binrep(const T& a)
{
    const char* beg = reinterpret_cast<const char*>(&a);
    const char* end = beg + sizeof(a);
    while (beg != end)
    {
        std::cout << std::bitset<CHAR_BIT>(*beg++) << ' ';
    }
    std::cout << '\n';
}
int main()
{
    int a, b;
    short c;
    a = 1;
    c = 2;
    b = a << 3;
    show_binrep(a);
    show_binrep(b);
    show_binrep(c);
    float f = 1;
    show_binrep(f);
}
// g++ test.cpp

6) type aliasing through reference

11) OOP type aliasing: type aliasing through pointer

Pointer to function

7) pointer to function conversion

10) OOP pointer to function conversion