When working with a C API that uses C-style inheritance, (taking advantage of the standard layout of C-structs), such as GLib, we usually use C-style casts to downcast:
struct base_object
{
int x;
int y;
int z;
};
struct derived_object
{
base_object base;
int foo;
int bar;
};
void func(base_object* b)
{
derived_object* d = (derived_object*) b; /* Downcast */
}
But if we're writing new C++ code that uses a C-API like this, should we continue to use C-style casts, or should we prefer C++ casts? If the latter, what type of C++ casts should we use to emulate C downcasting?
At first, I thought reinterpret_cast would be suitable:
derived_object* d = reinterpret_cast<derived_object*>(b);
However, I'm always wary of reinterpret_cast because the C++ standard guarantees very little about what will happen. It may be safer to use static_cast to void*:
derived_object* d = static_cast<derived_object*>(static_cast<void*>(b))
Of course, this is really cumbersome, making me think it's better to just use C-style casts in this case.
So what is the best practice here?
If you look at the specification for C-style casts in the C++ spec you'll find that cast notation is defined in terms of the other type conversion operators (dynamic_cast, static_cast, reinterpret_cast, const_cast), and in this case reinterpret_cast is used.
Additionally, reinterpret_cast gives more guarantees than is indicated by the answer you link to. The one you care about is:
§ 9.2/20: A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa.
If you want to use a cast notation I think using the C++ type conversion operators explicitly is best. However rather than littering casts all over the code you should probably write a function for each conversion (implemented using reinterpret_cast) and then use that.
derived_object *downcast_to_derived(base_object *b) {
return reinterpret_cast<derived_object*>(b);
}
However, I'm always wary of reinterpret_cast because the C++ standard
guarantees very little about what will happen.
C++-style casts are no less safe than C-style casts, because C-style cast is defined in terms of C++-style casts.
5.4.4 The conversions performed by
— a const_cast (5.2.11),
— a static_cast (5.2.9),
— a static_cast followed by a const_cast,
— a reinterpret_cast (5.2.10), or
— a reinterpret_cast followed by a const_cast,
can be performed using the cast notation of explicit type conversion.
[...]
If a conversion can be interpreted in more than one of the ways listed above, the interpretation that
appears first in the list is used, even if a cast resulting from that interpretation is ill-formed.
The sad answer is that you can't avoid casts in code like you written, because the compiler knows very little about relations between classes. Some way or another, you may want to refactor it (casts or classes or the code that uses them).
The bottom line is:
If you can, use proper inheritance.
If you can't, use reinterpret_cast.
new C++ code that uses a C-API like this
Don't write new C++ code in a C style, it doesn't make use of the C++ language features, and it also forces the user of your wrapper to use this same "C" style. Instead, create a proper C++ class that wraps the C API interface details and hides them behind a C++ class.
should we continue to use C-style casts
No
or should we prefer C++ casts
Yes, but only when you have to.
Use C++ inheritance and virtual accessor functions (probably). Please show how you plan to use the derived object in func, this may provide a better answer for you.
If func expects to use the methods of the derived object, then it should receive a derived object. If it expects to use the methods of a base_object, but the methods are somehow changed because the pointer is to a derived_object, then virtual functions are the C++ way to do this.
Also, you want to pass a reference to func, not a pointer.
dynamic_cast, requires certain conditions to be met:
http://www.cplusplus.com/doc/tutorial/typecasting/
If you are just converting struct ptrs to struct ptrs and you know what you want, then static_cast, or reinterpret_cast may be the best?
However, if you truly are interested in writing C++ code, then the casts should be your last and final resort, since there are better patterns. The two common situations I would consider casting are:
You are interfacing with some event passing mechanism that passes a generic base class to an event handler.
You have a container of objects. The container requires it to contain homogenous types (i.e every element contains the same "thing"), but you want to store different types in the container.
I think dynamic_cast is exactly what you want.
Related
This question concerns something I noticed in the C++ spec when I was trying to answer this earlier, intriguing question about C-style casts and type conversions.
The C++ spec talks about C-style casts in §5.4. It says that the cast notation will try the following casts, in this order, until one is found that is valid:
const_cast
static_cast
static_cast followed by const_cast
reinterpret_cast
reinterpret_cast followed by const_cast.
While I have a great intuitive idea of what it means to use a static_cast followed by a const_cast (for example, to convert a const Derived* to a Base* by going through a const_cast<Base*>(static_cast<const Base*>(expr))), I don't see any wording in the spec saying how, specifically, the types used in the static_cast/const_cast series are to be deduced. In the case of simple pointers it's not that hard, but as seen in the linked question the cast might succeed if an extra const is introduced in one place and removed in another.
Are there any rules governing how a compiler is supposed to determine what types to use in the casting chain? If so, where are they? If not, is this a defect in the language, or are there sufficient implicit rules to uniquely determine all the possible casts to try?
If not, is this a defect in the language, or are there sufficient implicit rules to uniquely determine all the possible casts to try?
What about constructing all types that can be cast to the target type using only const_cast, i.e. all "intermediate types"?
Given target type T, if static_cast doesn't work, identify all positions where one could add cv-qualifiers such that the resulting type can be cast back to T by const_cast1. Sketch of the algorithm: take the cv-decomposition ([conv.qual]/1) of T; each cvj can be augmented. If T is a reference, we can augment the referee type's cv-qualification.
Now add const volatile to all these places. Call the resulting type CT. Try static_casting the expression to CT instead. If that works, our cast chain is const_cast<T>(static_cast<CT>(e)).
If this doesn't work, there very probably is no conversion using static_cast followed by const_cast (I haven't delved into the deep corners of overload resolution (well I have, but not for this question)). But we could use brute force to repeatedly remove const/volatiles and check each type if we really wanted. So in theory, there is no ambiguity or underspecification; if there is some cast chain, it can be determined. In practice, the algorithm can be made very simple, because the "most cv-qualified" type we can construct from T is (assuredly) sufficient.
Almost a rehash of What's the difference between function(myVar) and (function)myVar?
But I want to know:
What is the name of these variants and are they 'bad'?
type(myVar) is constructor like syntax, but for a basic type is it the same as doing a C-style cast which is considered bad in C++?
(type)myVar this one certainly does seem to be a C-style cast and thus must be bad practice?
I've seen some instances where people replace things like (int)a with int(a) citing that the C-style version is bad/wrong yet the linked question says they're both the same!
What is the name of these variants
type(expr) is known as a function-style cast.
(type)(expr) is known as a C-style cast.
and are they 'bad'?
Yes. First off, both are semantically completely equivalent. They are “bad” because they aren’t safe – they might be equivalent to a static_cast, but equally a reinterpret_cast, and without knowing both type and the type of expr it’s impossible to say1. They also disregard access specifiers in some cases (C-style casts allow casting inside a private inheritance hierarchy). Furthermore, they are not as verbose as the explicit C++ style casts, which is a bad thing since casts are usually meant to stand out in C++.
1 Consider int(x): Depending on x’ type, this is either a static_cast (e.g. auto x = 4.2f;) or a reinterpret_cast (e.g. auto x = nullptr; on an architecture where int is large enough to hold a pointer).
The C++ casts static_cast, const_cast, reinterpret_cast have a template-like syntax, e.g.
long foo = 3;
int bar = static_cast<int>(foo);
I've looked in the Standard, and it says that casts are expressions, not template functions as I thought.
This left me wondering: under the hood, are these casts just templates with privileged status, or are they keywords that happen to borrow the template syntax?
are they keywords that happen to borrow the template syntax?
This. Casts are implemented differently depending on the context they are used in – in general, they cannot be implemented as functions. For instance, static_cast is sometimes only a compile-time operation, no code is emitted for it. But other times (in particular when invoking constructors, casting in a type hierarchy or converting between layout-incompatible primitive types) it requires a runtime operation.
That said, you can implement your own functions that resemble the standard cast syntax (boost::lexical_cast does that).
I've heard that the static_cast function should be preferred to C-style or simple function-style casting. Is this true? Why?
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.
One pragmatic tip: you can search easily for the static_cast keyword in your source code if you plan to tidy up the project.
In short:
static_cast<>() gives you a compile time checking ability, C-Style
cast doesn't.
static_cast<>() can be spotted easily
anywhere inside a C++ source code; in contrast, C_Style cast is harder to spot.
Intentions are conveyed much better using C++ casts.
More Explanation:
The static cast performs conversions between compatible types. It
is similar to the C-style cast, but is more restrictive. For example,
the C-style cast would allow an integer pointer to point to a char.
char c = 10; // 1 byte
int *p = (int*)&c; // 4 bytes
Since this results in a 4-byte pointer pointing to 1 byte of allocated
memory, writing to this pointer will either cause a run-time error or
will overwrite some adjacent memory.
*p = 5; // run-time error: stack corruption
In contrast to the C-style cast, the static cast will allow the
compiler to check that the pointer and pointee data types are
compatible, which allows the programmer to catch this incorrect
pointer assignment during compilation.
int *q = static_cast<int*>(&c); // compile-time error
Read more on:
What is the difference between static_cast<> and C style casting
and
Regular cast vs. static_cast vs. dynamic_cast
The question is bigger than just using whether static_cast<> or C-style casting because there are different things that happen when using C-style casts. The C++ casting operators are intended to make those different operations more explicit.
On the surface static_cast<> and C-style casts appear to be the same thing, for example when casting one value to another:
int i;
double d = (double)i; //C-style cast
double d2 = static_cast<double>( i ); //C++ cast
Both of those cast the integer value to a double. However when working with pointers things get more complicated. Some examples:
class A {};
class B : public A {};
A* a = new B;
B* b = (B*)a; //(1) what is this supposed to do?
char* c = (char*)new int( 5 ); //(2) that weird?
char* c1 = static_cast<char*>( new int( 5 ) ); //(3) compile time error
In this example (1) may be OK because the object pointed to by A is really an instance of B. But what if you don't know at that point in code what a actually points to?
(2) may be perfectly legal (you only want to look at one byte of the integer), but it could also be a mistake in which case an error would be nice, like (3).
The C++ casting operators are intended to expose these issues in the code by providing compile-time or run-time errors when possible.
So, for strict "value casting" you can use static_cast<>. If you want run-time polymorphic casting of pointers use dynamic_cast<>. If you really want to forget about types, you can use reintrepret_cast<>. And to just throw const out the window there is const_cast<>.
They just make the code more explicit so that it looks like you know what you were doing.
static_cast means that you can't accidentally const_cast or reinterpret_cast, which is a good thing.
Allows casts to be found easily in
your code using grep or similar
tools.
Makes it explicit what kind
of cast you are doing, and engaging
the compiler's help in enforcing it.
If you only want to cast away
const-ness, then you can use
const_cast, which will not allow you
to do other types of conversions.
Casts are inherently ugly -- you as
a programmer are overruling how the
compiler would ordinarily treat your
code. You are saying to the
compiler, "I know better than you."
That being the case, it makes sense
that performing a cast should be a
moderately painful thing to do, and
that they should stick out in your
code, since they are a likely source
of problems.
See Effective C++ Introduction
It's about how much type-safety you want to impose.
When you write (bar) foo (which is equivalent to reinterpret_cast<bar> foo if you haven't provided a type conversion operator) you are telling the compiler to ignore type safety, and just do as it's told.
When you write static_cast<bar> foo you are asking the compiler to at least check that the type conversion makes sense and, for integral types, to insert some conversion code.
EDIT 2014-02-26
I wrote this answer more than 5 years ago, and I got it wrong. (See comments.) But it still gets upvotes!
C Style casts are easy to miss in a block of code. C++ style casts are not only better practice; they offer a much greater degree of flexibility.
reinterpret_cast allows integral to pointer type conversions, however can be unsafe if misused.
static_cast offers good conversion for numeric types e.g. from as enums to ints or ints to floats or any data types you are confident of type. It does not perform any run time checks.
dynamic_cast on the other hand will perform these checks flagging any ambiguous assignments or conversions. It only works on pointers and references and incurs an overhead.
There are a couple of others but these are the main ones you will come across.
static_cast, aside from manipulating pointers to classes, can also be used to perform conversions explicitly defined in classes, as well as to perform standard conversions between fundamental types:
double d = 3.14159265;
int i = static_cast<int>(d);
What are the benefits of explicit type cast in C++ ?
They're more specific than the full general C-style casts. You don't give up quite as much type safety and the compiler can still double check some aspects of them for you.
They're easy to grep for if you want to try clean up your code.
The syntax intentionally mimics a templated function call. As a a result, you can "extend" the language by defining your own casts (e.g., Boost's lexical_cast).
Clarity in reading the code. There is not too much else of benefit, except for the cases where the compiler cannot infer the implicit cast at all, in which case you'd have to cast explicitly anyway.
The recommended way to cast in c++ is through dynamic_cast, static_cast, and the rest of those casting operators:
http://www.cplusplus.com/doc/tutorial/typecasting/
Notice that all typecasts are explicit in C++ and C. There are, in the language, no "implicit" typecasts. It's called "implicit conversions" and "explicit conversions", the latter of which are also called "casts".
Typecasts are most often used to inhibit warnings by the compiler. For instance if you have a signed and an unsigned value and compare them, the compiler usually warns you. If you know the comparison is correct, you can cast the operands to a signed type.
Another example is overload resolution, where type casts can be used to drive to the function to be called. For example to print out the address of a char, you can't just say cout << &c because that would try to interpret it as a C-Style String. Instead you would have to cast to void* before you print.
Often implicit conversions are superior to casts. Boost provides boost::implicit_cast to go by implicit conversions. For example the following relies on the implicit conversions of pointers to void*
cout << boost::implicit_cast<void*>(&c);
This has the benefit that it's only allowing conversions that are safe to do. Downcasts are not permitted, for example.
often used on void* to recover a implicitly known type. Especially common on embedded OS's for callbacks. This is a case where when you register for an event and maybe pass your "this" pointer as a context void*, then when the triggers it will pass you the void * context and you covert it back to the type the "this" pointer was.
Sometimes explicit casting avoids certain undesirable cirumstances by using explicit keyword.
class ExplicitExample
{
public:
ExplicitExample(int a){...}
}
ExplicitExample objExp = 'A';//No error.. call the integer constructor
Change as
explicit ExplicitExample(int a){ ... }
Now compile.
ExplicitExample objExp = 'A';
We get this error in VS2005.
error C2440: 'initializing' : cannot convert from 'char' to 'ExplicitExample'
Constructor for class 'ExplicitExample' is declared 'explicit'
To overcome this error, we have to declare as
ExplicitExample objExp = ExplicitExample('A');
It means as a programmer, we tell the compiler we know what we are calling. So compiler ignores this error.