What static_cast<> is actually doing
2014-01-15 19:57
351 查看
Introduction
Most programmers learn C before C++, and get used to C style casting. When writing C++, sometimes we may be confused about when to use static_cast<>and
when to use
reinterpret_cast<>.
In this article, I will illustrate what
static_cast<>actually
does, and will show some cases that will lead to errors.
Generic Types
Collapse | Copy
Code
float f = 12.3; float* pf = &f; // static cast<> // OK, n = 12 int n = static_cast<int>(f); // Error, types pointed to are unrelated //int* pn = static_cast<int*>(pf); // OK void* pv = static_cast<void*>(pf); // OK, but *pn2 is rubbish int* pn2 = static_cast<int*>(pv); // reinterpret_cast<> // Error, the compiler know you should // call static_cast<> //int i = reinterpret_cast<int>(f); // OK, but *pn is actually rubbish, same as *pn2 int* pi = reinterpret_cast<int*>(pf);
In short,
static_cast<>will
try to convert, e.g., float-to-integer, while
reinterpret_cast<>simply
changes the compiler's mind to reconsider that object as another type.
Pointer Types
Pointer casting is a bit complicated, we will use the following classes for the rest of the the article:Collapse | Copy
Code
class CBaseX { public: int x; CBaseX() { x = 10; } void foo() { printf("CBaseX::foo() x=%d\n", x); } }; class CBaseY { public: int y; int* py; CBaseY() { y = 20; py = &y; } void bar() { printf("CBaseY::bar() y=%d, *py=%d\n", y, *py); } }; class CDerived : public CBaseX, public CBaseY { public: int z; };
Case 1: Casting between unrelated classes
Collapse | Copy
Code
// Convert between CBaseX* and CBaseY* CBaseX* pX = new CBaseX(); // Error, types pointed to are unrelated // CBaseY* pY1 = static_cast<CBaseY*>(pX); // Compile OK, but pY2 is not CBaseX CBaseY* pY2 = reinterpret_cast<CBaseY*>(pX); // System crash!! // pY2->bar();
As we learnt in the generic types example,
static_cast<>will
fail if you try to cast an object to another unrelated class, while
reinterpret_cast<>will
always succeed by "cheating" the compiler to believe that the object is really that unrelated class.
Case 2: Casting to related classes
Collapse | Copy
Code
1. CDerived* pD = new CDerived(); 2. printf("CDerived* pD = %x\n", (int)pD); 3. 4. // static_cast<> CDerived* -> CBaseY* -> CDerived* // OK, implicit static_cast<> casting 5. CBaseY* pY1 = pD; 6. printf("CBaseY* pY1 = %x\n", (int)pY1); // OK, now pD1 = pD 7. CDerived* pD1 = static_cast<CDerived*>(pY1); 8. printf("CDerived* pD1 = %x\n", (int)pD1); 9. 10. // reinterpret_cast // OK, but pY2 is not CBaseY* 11. CBaseY* pY2 = reinterpret_cast<CBaseY*>(pD); 12. printf("CBaseY* pY2 = %x\n", (int)pY2); 13. 14. // unrelated static_cast<> 15. CBaseY* pY3 = new CBaseY(); 16. printf("CBaseY* pY3 = %x\n", (int)pY3); // OK, even pY3 is just a "new CBaseY()" 17. CDerived* pD3 = static_cast<CDerived*>(pY3); 18. printf("CDerived* pD3 = %x\n", (int)pD3);
Collapse | Copy
Code
---------------------- output --------------------------- CDerived* pD = 392fb8 CBaseY* pY1 = 392fbc CDerived* pD1 = 392fb8 CBaseY* pY2 = 392fb8 CBaseY* pY3 = 390ff0 CDerived* pD3 = 390fec
Noted that when
static_cast<>-ing
CDerived*to
CBaseY*(line
5), the result is
CDerived*offset by 4. To know what
static_cast<>is
actually doing, we have to take a look at the memory layout of
CDerived.
Memory Layout of CDerived
As shown in the diagram,
CDerived's memory layout contains two objects,
CBaseXand
CBaseY,
and the compiler knows this. Therefore, when you cast
CDerived*to
CBaseY*,
it adds the pointer by 4, and when you cast
CBaseYto
CDerived,
it subtracts the pointer by 4. However, you can do this even if it is not a
CDerived(line 14-18) [1].
Of course, the problem happens only if you have multiple inheritance.
static_cast<>and
reinterpret_cast<>make
no different if you are casting
CDerivedto
CBaseX.
Case 3: Casting back and forth between void*
Because any pointer can be cast to void*,
and
void*can be cast back to any pointer (true for both
static_cast<>and
reinterpret_cast<>),
errors may occur if not handled carefully.
Collapse | Copy
Code
CDerived* pD = new CDerived(); printf("CDerived* pD = %x\n", (int)pD); CBaseY* pY = pD; // OK, pY = pD + 4 printf("CBaseY* pY = %x\n", (int)pY); void* pV1 = pY; // OK, pV = pY printf("void* pV1 = %x\n", (int)pV1); // pD2 = pY, but we expect pD2 = pY - 4 CDerived* pD2 = static_cast<CDerived*>(pV1); printf("CDerived* pD2 = %x\n", (int)pD2); // System crash // pD2->bar();
Collapse | Copy
Code
---------------------- output --------------------------- CDerived* pD = 392fb8 CBaseY* pY = 392fbc void* pV1 = 392fbc CDerived* pD2 = 392fbc
Once we have cast the pointer to
void*,
we can't cast it back to the original class easily. In the above example, the only way to get back a
CDerived*from a
void*is
to cast it to a
CBaseY*and then to
CDerived*.
But if we are not sure whether it is
CBaseY*or
CDerived*,
then we have to use
dynamic_cast<>or
typeid[2].
Footnote
dynamic_cast<>,
on the other hand, can guard against casting a generic
CBaseY*to
CDerived*.
dynamic_cast<>requires
the classes to be "polymorphic", i.e., contains "virtual" function, and hence can't be
void*.
References
[MSDN] C++Language Reference -- Casting
Nishant Sivakumar, Casting
Basics - Use C++ casts in your VC++.NET programs
Juan Soulie, C++
Language Tutorial: Type Casting
History
3 Feb 2006: Initial version uploaded.
License
This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.A list of licenses authors might use can be found here
About the Author
Sam
NG
Hong Kong
No Biography provided
相关文章推荐
- What static_cast is actually doing
- What static_cast is actually doing
- What static_cast is actually doing
- <转> com.google.gson.internal.StringMap cannot be cast XXX(整理)
- C++类型转换运算符: static_cast<>,reinterpret_cast<>,dynamic_cast<>, const_cast<>
- the type List is not generic; it cannot be parameterized with arguments<T>解决办法
- C++ 中dynamic_cast<>的使用方法
- <The end tag "</c:forEach" is unbalanced>
- gvim中Mapping already in use: "<LocalLeader>is", mode "n"错误解决
- '<>' operator is not allowed for source level below 1.7
- web.xml is missing and <failOnMissingWebXml> is set to true
- <transmission> element is used to link actuators to joints
- Using C-style cast. Use reinterpret_cast<int*>(…) instead [readability/casting] [4]
- isstreamstream 头文件#include <sstream>
- static_cast<>揭密
- static_cast<>能够适用所有转型吗,有什么条件
- The SDK Build Tools revision<19.0.0> is too low for project*.*
- \sdk\include\wspiapi.h(47) : error C2265: '<Unknown>' : reference to a zero-sized array is illegal 解
- C++类型转换运算符之const_cast<newtype>(expression)
- C++ 中dynamic_cast<>的使用方法