For several reasons, I must use a struct defined in an extern C lib. I have simplified my code in order to make it readable.
Struct defined in C Lib
extern "C" {
typedef struct {
double (*_function)(double x);
} FuncR;
}
Cpp file containing class A
Class A {
public:
A();
void foo(double x); // Cannot be static since it uses m
void bar();
private:
double m; // non-static member by nature
};
void A::bar() {
FuncR f;
f._function = &A::foo;
};
The call f._function = &A::foo; generates the following error :
error C2440 : '=' : cannot convert from 'double (__thiscall A::*)(double)' to 'double(__cdecl*)(double)'
I have been searching for answers and apparently foomust be declared static. In my case it is not possible since it has to use non-static members...
Is there any trick to solve my problem ?
No, I don't think there's a trick to "fix" this.
The method call needs a this pointer, which the function pointer can't handle.
Often you can define a static "trampoline" function to get you into a method, but that requires that the outer layers (the C code, in this case) supports passing around e.g. a void * where you can store the this pointer.
Does the FuncR struct really need to be defined/used in C code? Can you use C++-style member function pointers?
How about this?...
class A;
struct classA_MemberFuncR {
double(A::*_function)(double);
};
class A {
public:
A() : m(1.234) {};
double foo(double x) {
std::cout << "In foo(" << x << "): this="
<< (void*)this << ", m=" << m << '\n';
return m+x;
}
void bar();
private:
double m; // non-static member by nature
};
void A::bar() {
classA_MemberFuncR fm;
fm._function = &A::foo;
double result = (this->*fm._function)(4.321);
std::cout << "Result is: " << result << std::endl;
};
[Comment added at this point:] Dang. Re-read OP's original post. (Supposedly we're stuck with the C struct's non-member function pointer.)
Hmm. On GCC I tried a whole lot of casting combinations and it never let me cast M::foo's address to much of anything else, so I wrote a runtime cast function template to force it to allow me to cast anything at all to any other type I want without complaining (Everyone: Please hold off on the screams of "that's not portable!" Sure it's portable... It's just how you use it that may or may not be portable!):
/*---- In a header file somewhere... ----*/
#include <stdarg.h>
template <typename ToTy, typename FromTy>
ToTy forceCast_helper(int dummy, ...);
template <typename ToTy, typename FromTy>
inline ToTy forceCast(FromTy obj) {
// ...has nothing to do with Star Wars!
return forceCast_helper<ToTy,FromTy>(1, obj);
}
/*---- In a source file somewhere... ----*/
template <typename ToTy, typename FromTy>
ToTy forceCast_helper(int dummy, ...) {
va_list args;
va_start(args, dummy);
ToTy ret = va_arg(args, ToTy);
va_end(args);
return ret;
}
Which allowed me to compile the following code without error:
typedef double(*doubleFuncDouble_t)(double);
typedef double(A::*doubleClassAMemberfuncDouble_t)(double);
f._function = forceCast<doubleFuncDouble_t>(&A::foo);
// and then call this->foo(4.321) with it from within A::bar() as follows...
(this->*(forceCast<doubleClassAMemberfuncDouble_t>(f._function)))(4.321);
Unfortunately when it ran, it segfaulted. Further investigation shows that, at least on GCC for 32-bit Linux for x86, sizeof(a member function pointer) is 8, while sizeof(a nonmember function pointer) is 4. When I changed the type of FuncR::_function to uint64_t, amazingly, the call succeeded. (I was surprised as well.)
So it seems regardless of any casting magic you might find that compiles without error, there's really no way at all you'd ever be able to squeeze a member function pointer into a nonmember function pointer, at least on GCC for 32-bit x86. And even if you could, that doesn't encapsulate the 'this' pointer, as 'unwind' mentioned in his post.
I think there's still hope, though.
unwind's post suggests a trampoline function but concedes that it would require separately passing the 'this' pointer & managing that in the C code as a void*. I'm assuming your C library is not modifiable? If so, you should still be able to do a trampoline without being able to pass a 'this' pointer through it, assuming you have a limited number of such function pointers you'll need to be specifying:
You could create an array of class A object pointers, sized to however many of these FuncR function pointer objects you'll be using:
A* arrayThatHoldsAObjPointers[8]; // assuming you only need 8 such FuncR func ptr objects
Then create that many physical static nonmember functions (each conveniently named with a suffix number corresponding with its associated array index), and in the body of each, make them call A::foo() via its associated 'A' object in the arrayThatHoldsAObjPointers:
double trampoline_0(double d) { return arrayThatHoldsAObjPointers[0]->foo(d); }
double trampoline_1(double d) { return arrayThatHoldsAObjPointers[1]->foo(d); }
...and so on...
Then when you need to set a FuncR object for use by your C library, give it the address of one of the trampolines, and at the same time, store the pointer to that A object in the associated arrayThatHoldsAObjPointers[] element. To make the code that sets these easier to use, you could also create an array of function pointers to the trampolines.
Is void* necessary apart from memory allocation related stuff in C++?
Can you give me an example?
Logging memory addresses
If you want to output a pointer using iostreams (e.g. for logging) then going via void* is the only way of ensuring operator<< hasn't been overloaded in some crazy way.
#include <iostream>
struct foo {
};
std::ostream& operator<<(std::ostream& out, foo*) {
return out<<"it's a trap!";
}
int main() {
foo bar;
foo *ptr = &bar;
std::cout << ptr << std::endl;
std::cout << static_cast<void*>(ptr) << std::endl;
}
Testing iostream status
iostreams overload operator void* as a status check so that syntax like if (stream) or while (stream) is a short hand way of testing the stream status.
Template meta programming
You might want to use void* with template metaprogramming sometimes as a reduced catch all, e.g. with SFINAE tricks, but more often than not there's a nicer way around it using a partial specialisation of one form or another.
Accessing most derived pointer
As Alf pointed out in the comments dynamic_cast<void*> is also useful for getting at the most derived type in a heirarchy, e.g.:
#include <iostream>
struct other {
virtual void func() = 0;
int c;
};
struct foo {
virtual void func() { std::cout << "foo" << std::endl; }
int a;
};
struct bar : foo, other {
virtual void func() { std::cout << "bar" << std::endl; }
int b;
};
namespace {
void f(foo *ptr) {
ptr->func();
std::cout << ptr << std::endl;
std::cout << dynamic_cast<void*>(ptr) << std::endl;
}
void g(other *ptr) {
ptr->func();
std::cout << ptr << std::endl;
std::cout << dynamic_cast<void*>(ptr) << std::endl;
}
}
int main() {
foo a;
bar b;
f(&a);
f(&b);
g(&b);
}
Gives:
foo
0xbfb815f8
0xbfb815f8
bar
0xbfb815e4
0xbfb815e4
bar
0xbfb815ec
0xbfb815e4
On my system.
Exceptions
§ 15.3.1 states:
The exception-declaration shall not denote a pointer or reference to
an incomplete type, other than void*, const void*, volatile void*, or
const volatile void*.
So it seems to be the only legal way of catching a pointer to an incomplete type is via void*. (Although I think there's possibly bigger issues if you actually needed to use that)
Legacy C uses
There are a lot of "legacy" C uses for void* for storing pointers to data without knowing what it is, but in new C++ code there is almost always a better way of expressing the same functionality.
No it isn't. It's only an idiom to refer to non typed memory
external libraries use them often (esp. in C).
in this case, i'll usually hide the use of these libraries (or the more dangerous portions). i hide them by writing interfaces around them (like wrapping). the interfaces i write serve to introduce type safety into the program. in this case, void* may be required, but at least it's hidden and restricted to a method or callback.
void* is frequently used for callbacks.
Callbacks are generally implemented in the C language using function
pointers and auxiliary user-defined data passed as a void pointer for
genericity.
[from here]
Of course, this is not type-safe, so people come up with ways to wrap this kind of thing, like here.
One situation where you might want to use void is when passing around data buffers, such as in this function:
void loadData(void* data, std::size_t size)
A lot of code passes buffers through a char-pointer instead of a void-pointer because, typically, buffers are read in 1 byte chunks, which happens to be the size that the C++ standard ensures a char to have.
However, using void-pointers is more generic. It's a way for the function to tell you, "Just give me some data, don't worry about how I'll read it". The function can then cast the pointer and read the data in chunks or whatever size it likes.
void * is used in C++ to express a pointer type to an unknown structure.
So I would use it whenever I have a pointer type where the code shouldn't know what is in there:
memory allocation
containers
...
Often void * mix quite well with template code to avoid template induced code bloat.
imagine you implement a
template <typename T>
class vector {
/*stuff */
};
you can then create a template specialisation for T * which uses void pointers so the code doesn't get duplicated.
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No C++ love when it comes to the "hidden features of" line of questions? Figured I would throw it out there. What are some of the hidden features of C++?
Most C++ programmers are familiar with the ternary operator:
x = (y < 0) ? 10 : 20;
However, they don't realize that it can be used as an lvalue:
(a == 0 ? a : b) = 1;
which is shorthand for
if (a == 0)
a = 1;
else
b = 1;
Use with caution :-)
You can put URIs into C++ source without error. For example:
void foo() {
http://stackoverflow.com/
int bar = 4;
...
}
Pointer arithmetics.
C++ programmers prefer to avoid pointers because of the bugs that can be introduced.
The coolest C++ I've ever seen though? Analog literals.
I agree with most posts there: C++ is a multi-paradigm language, so the "hidden" features you'll find (other than "undefined behaviours" that you should avoid at all cost) are clever uses of facilities.
Most of those facilities are not build-in features of the language, but library-based ones.
The most important is the RAII, often ignored for years by C++ developers coming from the C world. Operator overloading is often a misunderstood feature that enable both array-like behaviour (subscript operator), pointer like operations (smart pointers) and build-in-like operations (multiplying matrices.
The use of exception is often difficult, but with some work, can produce really robust code through exception safety specifications (including code that won't fail, or that will have a commit-like features that is that will succeed, or revert back to its original state).
The most famous of "hidden" feature of C++ is template metaprogramming, as it enables you to have your program partially (or totally) executed at compile-time instead of runtime. This is difficult, though, and you must have a solid grasp on templates before trying it.
Other make uses of the multiple paradigm to produce "ways of programming" outside of C++'s ancestor, that is, C.
By using functors, you can simulate functions, with the additional type-safety and being stateful. Using the command pattern, you can delay code execution. Most other design patterns can be easily and efficiently implemented in C++ to produce alternative coding styles not supposed to be inside the list of "official C++ paradigms".
By using templates, you can produce code that will work on most types, including not the one you thought at first. You can increase type safety,too (like an automated typesafe malloc/realloc/free). C++ object features are really powerful (and thus, dangerous if used carelessly), but even the dynamic polymorphism have its static version in C++: the CRTP.
I have found that most "Effective C++"-type books from Scott Meyers or "Exceptional C++"-type books from Herb Sutter to be both easy to read, and quite treasures of info on known and less known features of C++.
Among my preferred is one that should make the hair of any Java programmer rise from horror: In C++, the most object-oriented way to add a feature to an object is through a non-member non-friend function, instead of a member-function (i.e. class method), because:
In C++, a class' interface is both its member-functions and the non-member functions in the same namespace
non-friend non-member functions have no privileged access to the class internal. As such, using a member function over a non-member non-friend one will weaken the class' encapsulation.
This never fails to surprise even experienced developers.
(Source: Among others, Herb Sutter's online Guru of the Week #84: http://www.gotw.ca/gotw/084.htm )
One language feature that I consider to be somewhat hidden, because I had never heard about it throughout my entire time in school, is the namespace alias. It wasn't brought to my attention until I ran into examples of it in the boost documentation. Of course, now that I know about it you can find it in any standard C++ reference.
namespace fs = boost::filesystem;
fs::path myPath( strPath, fs::native );
Not only can variables be declared in the init part of a for loop, but also classes and functions.
for(struct { int a; float b; } loop = { 1, 2 }; ...; ...) {
...
}
That allows for multiple variables of differing types.
The array operator is associative.
A[8] is a synonym for *(A + 8). Since addition is associative, that can be rewritten as *(8 + A), which is a synonym for..... 8[A]
You didn't say useful... :-)
One thing that's little known is that unions can be templates too:
template<typename From, typename To>
union union_cast {
From from;
To to;
union_cast(From from)
:from(from) { }
To getTo() const { return to; }
};
And they can have constructors and member functions too. Just nothing that has to do with inheritance (including virtual functions).
C++ is a standard, there shouldn't be any hidden features...
C++ is a multi-paradigm language, you can bet your last money on there being hidden features. One example out of many: template metaprogramming. Nobody in the standards committee intended there to be a Turing-complete sublanguage that gets executed at compile-time.
Another hidden feature that doesn't work in C is the functionality of the unary + operator. You can use it to promote and decay all sorts of things
Converting an Enumeration to an integer
+AnEnumeratorValue
And your enumerator value that previously had its enumeration type now has the perfect integer type that can fit its value. Manually, you would hardly know that type! This is needed for example when you want to implement an overloaded operator for your enumeration.
Get the value out of a variable
You have to use a class that uses an in-class static initializer without an out of class definition, but sometimes it fails to link? The operator may help to create a temporary without making assumptins or dependencies on its type
struct Foo {
static int const value = 42;
};
// This does something interesting...
template<typename T>
void f(T const&);
int main() {
// fails to link - tries to get the address of "Foo::value"!
f(Foo::value);
// works - pass a temporary value
f(+Foo::value);
}
Decay an array to a pointer
Do you want to pass two pointers to a function, but it just won't work? The operator may help
// This does something interesting...
template<typename T>
void f(T const& a, T const& b);
int main() {
int a[2];
int b[3];
f(a, b); // won't work! different values for "T"!
f(+a, +b); // works! T is "int*" both time
}
Lifetime of temporaries bound to const references is one that few people know about. Or at least it's my favorite piece of C++ knowledge that most people don't know about.
const MyClass& x = MyClass(); // temporary exists as long as x is in scope
A nice feature that isn't used often is the function-wide try-catch block:
int Function()
try
{
// do something here
return 42;
}
catch(...)
{
return -1;
}
Main usage would be to translate exception to other exception class and rethrow, or to translate between exceptions and return-based error code handling.
Many know of the identity / id metafunction, but there is a nice usecase for it for non-template cases: Ease writing declarations:
// void (*f)(); // same
id<void()>::type *f;
// void (*f(void(*p)()))(int); // same
id<void(int)>::type *f(id<void()>::type *p);
// int (*p)[2] = new int[10][2]; // same
id<int[2]>::type *p = new int[10][2];
// void (C::*p)(int) = 0; // same
id<void(int)>::type C::*p = 0;
It helps decrypting C++ declarations greatly!
// boost::identity is pretty much the same
template<typename T>
struct id { typedef T type; };
A quite hidden feature is that you can define variables within an if condition, and its scope will span only over the if, and its else blocks:
if(int * p = getPointer()) {
// do something
}
Some macros use that, for example to provide some "locked" scope like this:
struct MutexLocker {
MutexLocker(Mutex&);
~MutexLocker();
operator bool() const { return false; }
private:
Mutex &m;
};
#define locked(mutex) if(MutexLocker const& lock = MutexLocker(mutex)) {} else
void someCriticalPath() {
locked(myLocker) { /* ... */ }
}
Also BOOST_FOREACH uses it under the hood. To complete this, it's not only possible in an if, but also in a switch:
switch(int value = getIt()) {
// ...
}
and in a while loop:
while(SomeThing t = getSomeThing()) {
// ...
}
(and also in a for condition). But i'm not too sure whether these are all that useful :)
Preventing comma operator from calling operator overloads
Sometimes you make valid use of the comma operator, but you want to ensure that no user defined comma operator gets into the way, because for instance you rely on sequence points between the left and right side or want to make sure nothing interferes with the desired action. This is where void() comes into game:
for(T i, j; can_continue(i, j); ++i, void(), ++j)
do_code(i, j);
Ignore the place holders i put for the condition and code. What's important is the void(), which makes the compiler force to use the builtin comma operator. This can be useful when implementing traits classes, sometimes, too.
Array initialization in constructor.
For example in a class if we have a array of int as:
class clName
{
clName();
int a[10];
};
We can initialize all elements in the array to its default (here all elements of array to zero) in the constructor as:
clName::clName() : a()
{
}
Oooh, I can come up with a list of pet hates instead:
Destructors need to be virtual if you intend use polymorphically
Sometimes members are initialized by default, sometimes they aren't
Local clases can't be used as template parameters (makes them less useful)
exception specifiers: look useful, but aren't
function overloads hide base class functions with different signatures.
no useful standardisation on internationalisation (portable standard wide charset, anyone? We'll have to wait until C++0x)
On the plus side
hidden feature: function try blocks. Unfortunately I haven't found a use for it. Yes I know why they added it, but you have to rethrow in a constructor which makes it pointless.
It's worth looking carefully at the STL guarantees about iterator validity after container modification, which can let you make some slightly nicer loops.
Boost - it's hardly a secret but it's worth using.
Return value optimisation (not obvious, but it's specifically allowed by the standard)
Functors aka function objects aka operator(). This is used extensively by the STL. not really a secret, but is a nifty side effect of operator overloading and templates.
You can access protected data and function members of any class, without undefined behavior, and with expected semantics. Read on to see how. Read also the defect report about this.
Normally, C++ forbids you to access non-static protected members of a class's object, even if that class is your base class
struct A {
protected:
int a;
};
struct B : A {
// error: can't access protected member
static int get(A &x) { return x.a; }
};
struct C : A { };
That's forbidden: You and the compiler don't know what the reference actually points at. It could be a C object, in which case class B has no business and clue about its data. Such access is only granted if x is a reference to a derived class or one derived from it. And it could allow arbitrary piece of code to read any protected member by just making up a "throw-away" class that reads out members, for example of std::stack:
void f(std::stack<int> &s) {
// now, let's decide to mess with that stack!
struct pillager : std::stack<int> {
static std::deque<int> &get(std::stack<int> &s) {
// error: stack<int>::c is protected
return s.c;
}
};
// haha, now let's inspect the stack's middle elements!
std::deque<int> &d = pillager::get(s);
}
Surely, as you see this would cause way too much damage. But now, member pointers allow circumventing this protection! The key point is that the type of a member pointer is bound to the class that actually contains said member - not to the class that you specified when taking the address. This allows us to circumvent checking
struct A {
protected:
int a;
};
struct B : A {
// valid: *can* access protected member
static int get(A &x) { return x.*(&B::a); }
};
struct C : A { };
And of course, it also works with the std::stack example.
void f(std::stack<int> &s) {
// now, let's decide to mess with that stack!
struct pillager : std::stack<int> {
static std::deque<int> &get(std::stack<int> &s) {
return s.*(pillager::c);
}
};
// haha, now let's inspect the stack's middle elements!
std::deque<int> &d = pillager::get(s);
}
That's going to be even easier with a using declaration in the derived class, which makes the member name public and refers to the member of the base class.
void f(std::stack<int> &s) {
// now, let's decide to mess with that stack!
struct pillager : std::stack<int> {
using std::stack<int>::c;
};
// haha, now let's inspect the stack's middle elements!
std::deque<int> &d = s.*(&pillager::c);
}
Another hidden feature is that you can call class objects that can be converted to function pointers or references. Overload resolution is done on the result of them, and arguments are perfectly forwarded.
template<typename Func1, typename Func2>
class callable {
Func1 *m_f1;
Func2 *m_f2;
public:
callable(Func1 *f1, Func2 *f2):m_f1(f1), m_f2(f2) { }
operator Func1*() { return m_f1; }
operator Func2*() { return m_f2; }
};
void foo(int i) { std::cout << "foo: " << i << std::endl; }
void bar(long il) { std::cout << "bar: " << il << std::endl; }
int main() {
callable<void(int), void(long)> c(foo, bar);
c(42); // calls foo
c(42L); // calls bar
}
These are called "surrogate call functions".
Hidden features:
Pure virtual functions can have implementation. Common example, pure virtual destructor.
If a function throws an exception not listed in its exception specifications, but the function has std::bad_exception in its exception specification, the exception is converted into std::bad_exception and thrown automatically. That way you will at least know that a bad_exception was thrown. Read more here.
function try blocks
The template keyword in disambiguating typedefs in a class template. If the name of a member template specialization appears after a ., ->, or :: operator, and that name has explicitly qualified template parameters, prefix the member template name with the keyword template. Read more here.
function parameter defaults can be changed at runtime. Read more here.
A[i] works as good as i[A]
Temporary instances of a class can be modified! A non-const member function can be invoked on a temporary object. For example:
struct Bar {
void modify() {}
}
int main (void) {
Bar().modify(); /* non-const function invoked on a temporary. */
}
Read more here.
If two different types are present before and after the : in the ternary (?:) operator expression, then the resulting type of the expression is the one that is the most general of the two. For example:
void foo (int) {}
void foo (double) {}
struct X {
X (double d = 0.0) {}
};
void foo (X) {}
int main(void) {
int i = 1;
foo(i ? 0 : 0.0); // calls foo(double)
X x;
foo(i ? 0.0 : x); // calls foo(X)
}
map::operator[] creates entry if key is missing and returns reference to default-constructed entry value. So you can write:
map<int, string> m;
string& s = m[42]; // no need for map::find()
if (s.empty()) { // assuming we never store empty values in m
s.assign(...);
}
cout << s;
I'm amazed at how many C++ programmers don't know this.
Putting functions or variables in a nameless namespace deprecates the use of static to restrict them to file scope.
Defining ordinary friend functions in class templates needs special attention:
template <typename T>
class Creator {
friend void appear() { // a new function ::appear(), but it doesn't
… // exist until Creator is instantiated
}
};
Creator<void> miracle; // ::appear() is created at this point
Creator<double> oops; // ERROR: ::appear() is created a second time!
In this example, two different instantiations create two identical definitions—a direct violation of the ODR
We must therefore make sure the template parameters of the class template appear in the type of any friend function defined in that template (unless we want to prevent more than one instantiation of a class template in a particular file, but this is rather unlikely). Let's apply this to a variation of our previous example:
template <typename T>
class Creator {
friend void feed(Creator<T>*){ // every T generates a different
… // function ::feed()
}
};
Creator<void> one; // generates ::feed(Creator<void>*)
Creator<double> two; // generates ::feed(Creator<double>*)
Disclaimer: I have pasted this section from C++ Templates: The Complete Guide / Section 8.4
void functions can return void values
Little known, but the following code is fine
void f() { }
void g() { return f(); }
Aswell as the following weird looking one
void f() { return (void)"i'm discarded"; }
Knowing about this, you can take advantage in some areas. One example: void functions can't return a value but you can also not just return nothing, because they may be instantiated with non-void. Instead of storing the value into a local variable, which will cause an error for void, just return a value directly
template<typename T>
struct sample {
// assume f<T> may return void
T dosomething() { return f<T>(); }
// better than T t = f<T>(); /* ... */ return t; !
};
Read a file into a vector of strings:
vector<string> V;
copy(istream_iterator<string>(cin), istream_iterator<string>(),
back_inserter(V));
istream_iterator
You can template bitfields.
template <size_t X, size_t Y>
struct bitfield
{
char left : X;
char right : Y;
};
I have yet to come up with any purpose for this, but it sure as heck surprised me.
One of the most interesting grammars of any programming languages.
Three of these things belong together, and two are something altogether different...
SomeType t = u;
SomeType t(u);
SomeType t();
SomeType t;
SomeType t(SomeType(u));
All but the third and fifth define a SomeType object on the stack and initialize it (with u in the first two case, and the default constructor in the fourth. The third is declaring a function that takes no parameters and returns a SomeType. The fifth is similarly declaring a function that takes one parameter by value of type SomeType named u.
Getting rid of forward declarations:
struct global
{
void main()
{
a = 1;
b();
}
int a;
void b(){}
}
singleton;
Writing switch-statements with ?: operators:
string result =
a==0 ? "zero" :
a==1 ? "one" :
a==2 ? "two" :
0;
Doing everything on a single line:
void a();
int b();
float c = (a(),b(),1.0f);
Zeroing structs without memset:
FStruct s = {0};
Normalizing/wrapping angle- and time-values:
int angle = (short)((+180+30)*65536/360) * 360/65536; //==-150
Assigning references:
struct ref
{
int& r;
ref(int& r):r(r){}
};
int b;
ref a(b);
int c;
*(int**)&a = &c;
The ternary conditional operator ?: requires its second and third operand to have "agreeable" types (speaking informally). But this requirement has one exception (pun intended): either the second or third operand can be a throw expression (which has type void), regardless of the type of the other operand.
In other words, one can write the following pefrectly valid C++ expressions using the ?: operator
i = a > b ? a : throw something();
BTW, the fact that throw expression is actually an expression (of type void) and not a statement is another little-known feature of C++ language. This means, among other things, that the following code is perfectly valid
void foo()
{
return throw something();
}
although there's not much point in doing it this way (maybe in some generic template code this might come handy).
The dominance rule is useful, but little known. It says that even if in a non-unique path through a base-class lattice, name-lookup for a partially hidden member is unique if the member belongs to a virtual base-class:
struct A { void f() { } };
struct B : virtual A { void f() { cout << "B!"; } };
struct C : virtual A { };
// name-lookup sees B::f and A::f, but B::f dominates over A::f !
struct D : B, C { void g() { f(); } };
I've used this to implement alignment-support that automatically figures out the strictest alignment by means of the dominance rule.
This does not only apply to virtual functions, but also to typedef names, static/non-virtual members and anything else. I've seen it used to implement overwritable traits in meta-programs.