錯誤：從'美孚*'轉換爲'無符號詮釋'失去精度

Question

我想轉換爲int（或unsigned int）的指針，無論我嘗試它不想工作。的空調風格

我試過static_cast<intptr_t>(obj)，reinterpret_cast<intptr_t>(obj)，以及各種組合投射，intptr_t的，unsigned int的，而我包括stdint.h。從我讀過的內容來看，我嘗試過的很多事情中的一件應該是有效的。是什麼賦予了？

我沒有理會包括代碼，因爲它正是我所描述的，但既然你問，我已經嘗試了所有這些再加上其他的組合：

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; 
    // ... 
} 

他們都給出確切的同樣的錯誤。

Answer 1

您可能在sizeof (Foo*) > sizeof (unsigned)的平臺上，或者您的編譯器設置爲警告有關不可移植的代碼。請注意，大多數64位編譯器，LP64和LLP64都屬於這一類。

沒有要求指針適合int。這是intptr_t的整點。

如果您使用的第三方庫在callbacls期間僅爲用戶上下文提供int，則可以將索引傳遞到查找表中，以便指針本身存儲在查找表中。這具有類型安全並且不會打破別名假設的額外好處。

編輯：適合我。 （Comeau "tryitout"是非常方便）

#include <stdint.h> 

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

Comeau C/C++ 4.3.10.1 (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.

在C89模式它也適用：

#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).