There is an interesting template presented on Wikipedia for Properties.
This template provides something interesting, in that it allows providing logic around member accesses. Building on this, we could easily build something like this:
struct Ranged {
ranged_property<float,0,1> unit_property;
};
Where the range of unit_property is enforced to be within [0,1].
How can we provide a similar functionality the depends on the hosting class' members? For example:
struct AdjustableRanged {
float max;
ranged_property<float,0,max> influenceable_property;
};
Where the range of influenceable_property is affected by the value of max. Keep in mind, the goal is for this kind of template to be recycled across many vastly different classes. Related concepts are mixins and decorators.
It can be done with macros... but I feel like there must be a better more idiomatic C++ solution.
Edited to add: I think this could be done by saving a reference to the member inside the ranged_property template... but that seems to be a complete waste of space for what would be effectively a constant value; ETA; A const reference may serve the purpose actually, however, I need to do the investigation.
Following up on our discussion in the comments, it seems the functionality can be achieved with a pointer-to-member template parameter like this (but see the caveats below):
#include <iostream>
template<typename C, typename T, T C::*m>
struct PrintMember {
C& obj;
PrintMember(C& obj) : obj(obj) {};
void print() { std::cout << "Member of containing class: " << obj.*m << std::endl; };
};
struct TestClass {
int data;
PrintMember<TestClass, int, &TestClass::data> pm;
TestClass() : pm(*this){};
};
int main()
{
TestClass tc;
tc.data = 5;
tc.pm.print();
}
This only demonstrates that it is possible to access members of the containing object. There are a few things that this approach doesn't solve:
If you really only want to access one member, it's not worth it, since you have to save a reference to *this in the PrintMember member to be able to dereference the pointer to member. So it doesn't actually solve the problem of having to store a reference. You could just as well pass a reference to the member variable itself in the constructor. However, if you need to access multiple members, this allows you to only store one reference (to *this) and still access all of them.
Specifying the template arguments of PrintMember and initializing the PrintMember member variable with *this in the constructor is tedious. Maybe, some clever template argument deduction can help here, but I haven't tried yet and I am not even sure it would get any simpler...
In some special cases, there might be dirty ways to get access to the "this" pointer of the enclosing class without saving it explicitly, like the answer using offsetof in this answer, but my feeling tells me you wanted something portable and less brittle...
I've recently spent a lot of time with javascript and am now coming back to C++. When I'm accessing a class member from a method I feed inclined to prefix it with this->.
class Foo {
int _bar;
public:
/* ... */
void setBar(int bar) {
this->_bar = bar;
// as opposed to
_bar = bar;
}
}
On reading, it saves me a brain cycle when trying to figure out where it's coming from.
Are there any reasons I shouldn't do this?
Using this-> for class variables is perfectly acceptable.
However, don't start identifiers with an underscore, or include any identifiers with double underscore __ anywhere. There are some classes of reserved symbols that are easy to hit if you violate either of these two rules of thumb. (In particular, _IdentifierStartingWithACapital is reserved by the standard for compilers).
In principle, accessing members via this-> is a coding style that can help in making things clearer, but it seems to be a matter of taste.
However, you also seem to use prefixing members with _ (underscore). I would say that is too much, you should go for either of the two styles.
Are there any reasons I shouldn't do this?
Yes, there is a reason why you shouldn't do this.
Referencing a member variable with this-> is strictly required only when a name has been hidden, such as with:
class Foo
{
public:
void bang(int val);
int val;
};
void Foo::bang(int val)
{
val = val;
}
int main()
{
Foo foo;
foo.val = 42;
foo.bang(84);
cout << foo.val;
}
The output of this program is 42, not 84, because in bang the member variable has been hidden, and val = val results in a no-op. In this case, this-> is required:
void Foo::bang(int val)
{
this->val = val;
}
In other cases, using this-> has no effect, so it is not needed.
That, in itself, is not a reason not to use this->. The maintennance of such a program is however a reason not to use this->.
You are using this-> as a means of documentation to specify that the vairable that follows is a member variable. However, to most programmers, that's not what usign this-> actually documents. What using this-> documents is:
There is a name that's been hidden here, so I'm using a special
technique to work around that.
Since that's not what you wanted to convey, your documentation is broken.
Instead of using this-> to document that a name is a member variable, use a rational naming scheme consistently where member variables and method parameters can never be the same.
Edit Consider another illustration of the same idea.
Suppose in my codebase, you found this:
int main()
{
int(*fn)(int) = pingpong;
(fn)(42);
}
Quite an unusual construct, but being a skilled C++ programmer, you see what's happening here. fn is a pointer-to-function, and being assigned the value of pingpong, whatever that is. And then the function pointed to by pingpong is being called with the singe int value 42. So, wondering why in the world you need such a gizmo, you go looking for pingpong and find this:
static int(*pingpong)(int) = bangbang;
Ok, so what's bangbang?
int bangbang(int val)
{
cout << val;
return val+1;
}
"Now, wait a sec. What in the world is going on here? Why do we need to create a pointer-to-function and then call through that? Why not just call the function? Isn't this the same?"
int main()
{
bangbang(42);
}
Yes, it is the same. The observable effects are the same.
Wondering if that's really all there is too it, you see:
/* IMPLEMENTATION NOTE
*
* I use pointers-to-function to call free functions
* to document the difference between free functions
* and member functions.
*/
So the only reason we're using the pointer-to-function is to show that the function being called is a free function
and not a member function.
Does that seem like just a "matter of style" to you? Because it seems like insanity to me.
Here you will find:
Unless a class member name is hidden, using the class member name is equivalent to using the class member name with the this pointer and the class member access operator (->).
I think you do this backwards. You want the code to assure you that what happens is exactly what is expected.
Why add extra code to point out that nothing special is happening? Accessing class members in the member functions happen all the time. That's what would be expected. It would be much better to add extra info when it is not the normal things that happen.
In code like this
class Foo
{
public:
void setBar(int NewBar)
{ Bar = NewBar; }
you ask yourself - "Where could the Bar come from?".
As this is a setter in a class, what would it set if not a class member variable?! If it wasn't, then there would be a reason to add a lot of info about what's actually going on here!
Since you are already using a convention to signify that an identifer is a data member (although not one I would recommend), adding this-> is simply redundant in almost all cases.
This is a somewhat subjective question obvously. this-> seems much more python-idiomatic than C++-idiomatic. There are only a handful of cases in C++ where the leading this-> is required, dealing with names in parent template classes. In general if your code is well organized it will be obvious to the reader that it's a member or local variable (globals should just be avoided), and reducing the amount to be read may reduce complexity. Additionally you can use an optional style (I like trailing _) to indicate member variables.
It doesn't actually harm anything, but programmers experienced with OO will see it and find it odd. It's similarly surprising to see "yoda conditionals," ie if (0 == x).
Suppose I have an object that has a member variable that is of some template type. So, in the declaration of the class, there would be something like this:
// This is just the declaration of bar which is a member of some class.
templatizedType<Foo> bar;
Now, when I want to initialize bar why do I have to do
// This is the initialization. Note that I am assuming that templatizedType has a
// constructor that takes an argument of type T*. Presumably, this is happening
// somewhere inside whatever class has declared bar as a member.
templatizedType<Foo> bar(new Foo());
instead of simply
bar(new Foo());
EDIT(trying to clarify): Essentially, it seems to me that the type of bar (including the parametrized type) is already spelled out in it's declaration as a member of the class and thus should not require a repeat upon initialization.
If none of this makes sense, let me know (I discovered this mostly through trial-and-error and some helpful people on IRC, so if my understanding of what is going on here is wrong, help with that would also be greatly appreciated.)
templatizedType<Foo> bar;
calls the default constructor, while
templatizedType<Foo> bar( new Foo() );
calls the constructor taking a Foo* as first argument
To construct an object, you have to write the type.
That is why,
bar( new Foo() )
does not call the constructor of templatizedType, but instead calls a method on an already constructed object of that type. This method could be, for instance:
void operator()( Foo* )
hope this helps..
Since C++ is a strongly typed language, it wants to make sure that this "bar" thing you are referring to really is something that can accept "new Foo()" therefore you need to give it a type, as in the line:
templatizedType<Foo> bar(new Foo());
Also if you just say
bar(new Foo());
Who is to say this isn't a function bar() vs a variable declaration?
Are you sure that you're not just overloading the name bar as a local variable in your constructor i.e. if your class is called A are you doing
A::A()
{
templatizedType<Foo> bar(new Foo());
}
instead of
A::A()
: bar(new Foo())
{
}
The reason is because of the fact that resolving overloads to templated functions is difficult already, without having to deal with the nastiness that is partial specialization and template members of templates (I know it sounds confusing. That's because it's confusing.)
Basically, determining what to call when you have this:
void foo (int i);
template <typename T> void foo (T t);
is a lot easier than figuring out what to call when you have this:
template <typename T> class foo {
foo (T t);
template <typename U> foo (U u);
};
The possibilities for constructors are too immense to be able to sort out through some heuristic like is done for function templates, so the standard simply doesn't even try (it's the right call, in my opinion). Instead, templated types can provide make_**** functions (make_pair comes to mind) which serve as templated constructors.
I think at best you could hope for a declaration such as
TemplatizedType<> bar(new Foo());
but this kind of deduction is not going to happen. I suppose the greatest reason would be that the compiler first needs to know which TemplatizedType to instantiate, before it can even check to see what constructors there are.
With C++0x you will be able to combine make_xxx functions that deduce template arguments with the auto keyword:
auto bar = make_templatized_type(new Foo());
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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.