Click here to Skip to main content
15,867,686 members
Articles / Programming Languages / C++
Article

What static_cast<> is actually doing

Rate me:
Please Sign up or sign in to vote.
4.71/5 (57 votes)
2 Feb 20062 min read 188.6K   81   9
An article discussing about static_cast<> and reinterpret_cast<>.

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

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:

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

// 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

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);
---------------------- 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

Class Memory Layout

As shown in the diagram, CDerived's memory layout contains two objects, CBaseX and CBaseY, and the compiler knows this. Therefore, when you cast CDerived* to CBaseY*, it adds the pointer by 4, and when you cast CBaseY to 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 CDerived to 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.

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();
---------------------- 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

  1. dynamic_cast<>, on the other hand, can guard against casting a generic CBaseY* to CDerived*.
  2. dynamic_cast<> requires the classes to be "polymorphic", i.e., contains "virtual" function, and hence can't be void*.

References

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


Written By
Hong Kong Hong Kong
This member has not yet provided a Biography. Assume it's interesting and varied, and probably something to do with programming.

Comments and Discussions

 
QuestionExcellent article Pin
viewlucc10-Nov-15 12:55
viewlucc10-Nov-15 12:55 
Thanks!
GeneralRegarding access to Derived class Data. Pin
Sameerkumar Namdeo9-Mar-07 16:59
Sameerkumar Namdeo9-Mar-07 16:59 
GeneralRe: Regarding access to Derived class Data. Pin
Johann Gerell8-Jun-08 22:50
Johann Gerell8-Jun-08 22:50 
General5 Pin
Stephen Hewitt4-May-06 21:58
Stephen Hewitt4-May-06 21:58 
Generalgood article Pin
xiedong7-Feb-06 20:23
xiedong7-Feb-06 20:23 
GeneralI like the article... Pin
Jun Du6-Feb-06 6:07
Jun Du6-Feb-06 6:07 
GeneralRe: I like the article... Pin
Sam NG6-Feb-06 14:39
Sam NG6-Feb-06 14:39 
GeneralMy kind of article Pin
ANewCoder3-Feb-06 5:01
ANewCoder3-Feb-06 5:01 
GeneralNice! Pin
Emilio Garavaglia2-Feb-06 22:14
Emilio Garavaglia2-Feb-06 22:14 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.