错误：从“Foo *”转换为“unsigned int”会丢失精度 [英] error: cast from 'Foo*' to 'unsigned int' loses precision

问题描述

我试图把一个指针指向一个int（或unsigned int），无论我尝试它不想工作。

我试过了 static_cast< intptr_t>（obj）， reinterpret_cast< intptr_t& （obj），以及C风格转换的各种组合 intptr_t '， unsigned int ，我包括stdint.h。从我读过的，我尝试过的许多事情之一应该工作。什么给了？

我没有打扰包含代码，因为它正是我所描述的，但是自从你问以后，我试过所有这些加上其他组合： / p>

  void myfunc（Foo * obj）
 {
 // ... 
 uintptr_t temp = reinterpret_cast< uintptr_t>（obj）; 
 uintptr_t temp = static_cast< uintptr_t>（obj）; 
 uintptr_t temp =（uintptr_t）obj; 
 intptr_t temp = reinterpret_cast< intptr_t>（obj）; 
 intptr_t temp = static_cast< intptr_t>（obj）; 
 intptr_t temp =（intptr_t）obj; 
 unsigned int temp = reinterpret_cast< unsigned int>（obj）; 
 unsigned int temp = static_cast< unsigned int>（obj）; 
 unsigned int temp =（unsigned int）obj; 
 // ... 
} 
  

。

解决方案

您可以在 sizeof（Foo *）& sizeof（unsigned），或者您的编译器设置为关于非可移植代码的警告。请注意，大多数64位编译器，LP64和LLP64都属于此类别。

不需要指针适合 int 。这是 intptr_t 的整个点。

如果您使用的第三方库只提供 int 对于callbacls中的用户上下文，可以将索引传递到查找表中，因此指针本身存储在查找表中。这有额外的好处是类型安全，不打破别名假设。

编辑：适用于我。 （ Comeautryitout非常方便）

  #include< stdint.h> 
 
 void myfunc（class Foo * obj）
 {
 uintptr_t temp = reinterpret_cast< uintptr_t>（obj）; 
} 
  

Comeau C / C ++ 4.3.10.1 6 2008
11:28:09）for ONLINE_EVALUATION_BETA2
版权所有1988-2008 Comeau Computing。
保留所有权利。 MODE：strict
errors C ++ C ++ 0x_extensions

ComeauTest.c第5行：警告：
声明了变量temp从不
引用
uintptr_t temp = reinterpret_cast（obj）; reinterpret_cast（obj）;

在严格模式下，记住，Comeau在线编译器没有链接）。
编译时启用了C ++ 0x扩展。

在C89模式下，它也可以工作：

  #include< stdint.h> 
 
 void myfunc（struct Foo * obj）
 {
 uintptr_t temp =（uintptr_t）obj; 
} 
  

Comeau C / C ++ 4.3.10.1 6 2008
11:28:09）for ONLINE_EVALUATION_BETA2
版权所有1988-2008 Comeau Computing。
保留所有权利。 MODE：strict
错误C90

ComeauTest.c，第3行：警告：
声明在
函数外部不可见void myfunc（struct Foo *
obj）
^

ComeauTest.c，第5行：warning：
variabletemp 被声明，但从不
引用
uintptr_t temp =（uintptr_t）obj;
^

在严格模式下，使用-tused，Compile
成功（但请记住，Comeau
在线编译器未链接） 。

I'm trying to cast a pointer to an int (or unsigned int) and no matter what I try it doesn't want to work.

I've tried static_cast<intptr_t>(obj), reinterpret_cast<intptr_t>(obj), and various combinations of C style casts, intptr_t's, unsigned int's, and I'm including stdint.h. From what I've read, one of the many things I've tried should work. What gives?

I didn't bother including the code because it's exactly what I described, but since you asked, I've tried all of these plus other combinations:

void myfunc(Foo* obj)
{
    // ...
    uintptr_t temp = reinterpret_cast<uintptr_t>(obj);
    uintptr_t temp = static_cast<uintptr_t>(obj);
    uintptr_t temp = (uintptr_t)obj;
    intptr_t temp = reinterpret_cast<intptr_t>(obj);
    intptr_t temp = static_cast<intptr_t>(obj);
    intptr_t temp = (intptr_t)obj;
    unsigned int temp = reinterpret_cast<unsigned int>(obj);
    unsigned int temp = static_cast<unsigned int>(obj);
    unsigned int temp = (unsigned int)obj;
    // ...
}

They all give the exact same error.

解决方案

You're either on a platform where sizeof (Foo*) > sizeof (unsigned), or your compiler is set to warn about non-portable code. Note that most 64-bit compilers, both LP64 and LLP64, fall into this category.

There's no requirement that a pointer fit in an int. That's the whole point of intptr_t.

If you're using a third-party library that provides only a int for user-context during callbacls, you could pass an index into a lookup table, so the pointer itself is stored in the lookup table. This has the additional benefit of being type-safe and not breaking aliasing assumptions.

EDIT: Works for me. (Comeau "tryitout" is very handy)

#include <stdint.h>

void myfunc(class Foo* obj)
{
    uintptr_t temp = reinterpret_cast<uintptr_t>(obj);
}

Comeau C/C++ 4.3.10.1 (Oct 6 2008 11:28:09) for ONLINE_EVALUATION_BETA2 Copyright 1988-2008 Comeau Computing. All rights reserved. MODE:strict errors C++ C++0x_extensions

"ComeauTest.c", line 5: warning: variable "temp" was declared but never referenced uintptr_t temp = reinterpret_cast(obj);reinterpret_cast(obj);

In strict mode, with -tused, Compile succeeded (but remember, the Comeau online compiler does not link). Compiled with C++0x extensions enabled.

In C89 mode it also works:

#include <stdint.h>

void myfunc(struct Foo* obj)
{
    uintptr_t temp = (uintptr_t)obj;
}

Comeau C/C++ 4.3.10.1 (Oct 6 2008 11:28:09) for ONLINE_EVALUATION_BETA2 Copyright 1988-2008 Comeau Computing. All rights reserved. MODE:strict errors C90

"ComeauTest.c", line 3: warning: declaration is not visible outside of function void myfunc(struct Foo* obj) ^

"ComeauTest.c", line 5: warning: variable "temp" was declared but never referenced uintptr_t temp = (uintptr_t)obj; ^

In strict mode, with -tused, Compile succeeded (but remember, the Comeau online compiler does not link).

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