The main reason is that classic C casts make no distinction between what we call static_cast<>()
, reinterpret_cast<>()
, const_cast<>()
, and dynamic_cast<>()
. These four things are completely different.
A static_cast<>()
is usually safe. There is a valid conversion in the language, or an appropriate constructor that makes it possible. The only time it's a bit risky is when you cast down to an inherited class; you must make sure that the object is actually the descendant that you claim it is, by means external to the language (like a flag in the object). A dynamic_cast<>()
is safe as long as the result is checked (pointer) or a possible exception is taken into account (reference).
A reinterpret_cast<>()
(or a const_cast<>()
) on the other hand is always dangerous. You tell the compiler: "trust me: I know this doesn't look like a foo
(this looks as if it isn't mutable), but it is".
The first problem is that it's almost impossible to tell which one will occur in a C-style cast without looking at large and disperse pieces of code and knowing all the rules.
Let's assume these:
class CDerivedClass : public CMyBase {...};
class CMyOtherStuff {...} ;
CMyBase *pSomething; // filled somewhere
Now, these two are compiled the same way:
CDerivedClass *pMyObject;
pMyObject = static_cast<CDerivedClass*>(pSomething); // Safe; as long as we checked
pMyObject = (CDerivedClass*)(pSomething); // Same as static_cast<>
// Safe; as long as we checked
// but harder to read
However, let's see this almost identical code:
CMyOtherStuff *pOther;
pOther = static_cast<CMyOtherStuff*>(pSomething); // Compiler error: Can't convert
pOther = (CMyOtherStuff*)(pSomething); // No compiler error.
// Same as reinterpret_cast<>
// and it's wrong!!!
As you can see, there is no easy way to distinguish between the two situations without knowing a lot about all the classes involved.
The second problem is that the C-style casts are too hard to locate. In complex expressions it can be very hard to see C-style casts. It is virtually impossible to write an automated tool that needs to locate C-style casts (for example a search tool) without a full blown C++ compiler front-end. On the other hand, it's easy to search for "static_cast<" or "reinterpret_cast<".
pOther = reinterpret_cast<CMyOtherStuff*>(pSomething);
// No compiler error.
// but the presence of a reinterpret_cast<> is
// like a Siren with Red Flashing Lights in your code.
// The mere typing of it should cause you to feel VERY uncomfortable.
That means that, not only are C-style casts more dangerous, but it's a lot harder to find them all to make sure that they are correct.
Read it backwards (as driven by Clockwise/Spiral Rule):
int*
- pointer to int
int const *
- pointer to const int
int * const
- const pointer to int
int const * const
- const pointer to const int
Now the first const
can be on either side of the type so:
const int *
== int const *
const int * const
== int const * const
If you want to go really crazy you can do things like this:
int **
- pointer to pointer to int
int ** const
- a const pointer to a pointer to an int
int * const *
- a pointer to a const pointer to an int
int const **
- a pointer to a pointer to a const int
int * const * const
- a const pointer to a const pointer to an int
- ...
And to make sure we are clear on the meaning of const
:
int a = 5, b = 10, c = 15;
const int* foo; // pointer to constant int.
foo = &a; // assignment to where foo points to.
/* dummy statement*/
*foo = 6; // the value of a canĀ“t get changed through the pointer.
foo = &b; // the pointer foo can be changed.
int *const bar = &c; // constant pointer to int
// note, you actually need to set the pointer
// here because you can't change it later ;)
*bar = 16; // the value of c can be changed through the pointer.
/* dummy statement*/
bar = &a; // not possible because bar is a constant pointer.
foo
is a variable pointer to a constant integer. This lets you change what you point to but not the value that you point to. Most often this is seen with C-style strings where you have a pointer to a const char
. You may change which string you point to but you can't change the content of these strings. This is important when the string itself is in the data segment of a program and shouldn't be changed.
bar
is a constant or fixed pointer to a value that can be changed. This is like a reference without the extra syntactic sugar. Because of this fact, usually you would use a reference where you would use a T* const
pointer unless you need to allow NULL
pointers.
Best Answer
In
you sort-of implicitly re-declare two integer variables you have declared before. You shouldn't do that. Instead, simply use them:
Now still, this is probably not what you meant. You probably meant to say something like this: