What is the general idea of a delegate in C++? What are they, how are they used and what are they used for?
I'd like to first learn about them in a 'black box' way, but a bit of information on the guts of these things would be great too.
This is not C++ at its purest or cleanest, but I notice that the codebase where I work has them in abundance. I'm hoping to understand them enough, so I can just use them and not have to delve into the horrible nested template awfulness.
These two The Code Project articles explain what I mean but not particularly succinctly:
Member Function Pointers and the Fastest Possible C++ Delegates
The Impossibly Fast C++ Delegates
You have an incredible number of choices to achieve delegates in C++. Here are the ones that came to my mind.
Option 1 : functors:
A function object may be created by implementing operator()
struct Functor
{
// Normal class/struct members
int operator()(double d) // Arbitrary return types and parameter list
{
return (int) d + 1;
}
};
// Use:
Functor f;
int i = f(3.14);
Option 2: lambda expressions (C++11 only)
// Syntax is roughly: [capture](parameter list) -> return type {block}
// Some shortcuts exist
auto func = [](int i) -> double { return 2*i/1.15; };
double d = func(1);
Option 3: function pointers
int f(double d) { ... }
typedef int (*MyFuncT) (double d);
MyFuncT fp = &f;
int a = fp(3.14);
Option 4: pointer to member functions (fastest solution)
See Fast C++ Delegate (on The Code Project).
struct DelegateList
{
int f1(double d) { }
int f2(double d) { }
};
typedef int (DelegateList::* DelegateType)(double d);
DelegateType d = &DelegateList::f1;
DelegateList list;
int a = (list.*d)(3.14);
Option 5: std::function
(or boost::function if your standard library doesn't support it). It is slower, but it is the most flexible.
#include <functional>
std::function<int(double)> f = [can be set to about anything in this answer]
// Usually more useful as a parameter to another functions
Option 6: binding (using std::bind)
Allows setting some parameters in advance, convenient to call a member function for instance.
struct MyClass
{
int DoStuff(double d); // actually a DoStuff(MyClass* this, double d)
};
std::function<int(double d)> f = std::bind(&MyClass::DoStuff, this, std::placeholders::_1);
// auto f = std::bind(...); in C++11
Option 7: templates
Accept anything as long as it matches the argument list.
template <class FunctionT>
int DoSomething(FunctionT func)
{
return func(3.14);
}
A delegate is a class that wraps a pointer or reference to an object instance, a member method of that object's class to be called on that object instance, and provides a method to trigger that call.
Here's an example:
template <class T>
class CCallback
{
public:
typedef void (T::*fn)( int anArg );
CCallback(T& trg, fn op)
: m_rTarget(trg)
, m_Operation(op)
{
}
void Execute( int in )
{
(m_rTarget.*m_Operation)( in );
}
private:
CCallback();
CCallback( const CCallback& );
T& m_rTarget;
fn m_Operation;
};
class A
{
public:
virtual void Fn( int i )
{
}
};
int main( int /*argc*/, char * /*argv*/ )
{
A a;
CCallback<A> cbk( a, &A::Fn );
cbk.Execute( 3 );
}
The need for C++ delegate implementations are a long lasting embarassment to the C++ community.
Every C++ programmer would love to have them, so they eventually use them despite the facts that:
std::function() uses heap operations (and is out of reach for serious embedded programming).
All other implementations make concessions towards either portability or standard conformity to larger or lesser degrees (please verify by inspecting the various delegate implementations here and on codeproject). I have yet to see an implementation which does not use wild reinterpret_casts, Nested class "prototypes" which hopefully produce function pointers of the same size as the one passed in by the user, compiler tricks like first forward declare, then typedef then declare again, this time inheriting from another class or similar shady techniques. While it is a great accomplishment for the implementers who built that, it is still a sad testimoney on how C++ evolves.
Only rarely is it pointed out, that now over 3 C++ standard revisions, delegates were not properly addressed. (Or the lack of language features which allow for straightforward delegate implementations.)
With the way C++11 lambda functions are defined by the standard (each lambda has anonymous, different type), the situation has only improved in some use cases. But for the use case of using delegates in (DLL) library APIs, lambdas alone are still not usable. The common technique here, is to first pack the lambda into a std::function and then pass it across the API.
Very simply, a delegate provides functionality for how a function pointer SHOULD work. There are many limitations of function pointers in C++. A delegate uses some behind-the-scenes template nastyness to create a template-class function-pointer-type-thing that works in the way you might want it to.
ie - you can set them to point at a given function and you can pass them around and call them whenever and wherever you like.
There are some very good examples here:
http://www.codeproject.com/Articles/7150/Member-Function-Pointers-and-the-Fastest-Possible
http://www.codeproject.com/Articles/11015/The-Impossibly-Fast-C-Delegates
http://www.codeproject.com/Articles/13287/Fast-C-Delegate
An option for delegates in C++ that is not otherwise mentioned here is to do it C style using a function ptr and a context argument. This is probably the same pattern that many asking this question are trying to avoid. But, the pattern is portable, efficient, and is usable in embedded and kernel code.
class SomeClass
{
in someMember;
int SomeFunc( int);
static void EventFunc( void* this__, int a, int b, int c)
{
SomeClass* this_ = static_cast< SomeClass*>( this__);
this_->SomeFunc( a );
this_->someMember = b + c;
}
};
void ScheduleEvent( void (*delegateFunc)( void*, int, int, int), void* delegateContext);
...
SomeClass* someObject = new SomeObject();
...
ScheduleEvent( SomeClass::EventFunc, someObject);
...
Windows Runtime equivalent of a function object in standard C++. One can use the whole function as a parameter (actually that is a function pointer). It is mostly used in conjunction with events. The delegate represents a contract that event handlers much fulfill. It facilitate how a function pointer can work for.
the question is how to get pointer to self inside method of a class without touching this:
class Foo
{
int a, b, c;
void Print();
};
This way in common compiler I can do this refering to first data field:
void Foo::Print()
{
cout << &a; // == this
}
But are there any ways to do this without data members when only function exists?
class Foo2
{
void Print();
};
p.s. don't even ask me why do I need this :)
For a POD class with at least one data member, the address of the class-type object is the same as the address of its first data member. This is because there can be no unnamed padding bytes before the first data member of a POD struct type. [In C++11, the rules are a bit different; I believe that this is true for all standard layout class types. I am not entirely familiar with the rules, however.]
For any other class type, there is no way to do this.
class Foo2
{
void Print();
};
#define OBJADDR(x) (th##x##s)
void Foo2::Print()
{
int i; // red herring, but the name MUST be `i`
std::cout << OBJADDR(i);
i; // to get rid of compiler warning about unused local variable
};
If your class has no data members then technically, instances of the class don't even exist. For convenience only, they are required to have size of at least 1 byte and have unique this pointers. If you inherit from a class with no data members, its size collapses to 0 and it disappears.
So no, there isn't any way to get the this pointer without using the this pointer. A less pragmatic version of C++ wouldn't even have this pointers for these objects, as they would have size 0.
You can probably hack it using the offsetof macro
void Foo::Print()
{
cout << static_cast<char *>(&a) - offsetof(Foo, a);
}
if you can't safely assume &a is the start.
So ... why do you need to do this?
I have a C library that has types like this:
typedef struct {
// ...
} mytype;
mytype *mytype_new() {
mytype *t = malloc(sizeof(*t));
// [Initialize t]
return t;
}
void mytype_dosomething(mytype *t, int arg);
I want to provide C++ wrappers to provide a better syntax. However, I want to avoid the complication of having a separately-allocated wrapper object. I have a relatively complicated graph of objects whose memory-management is already more complicated than I would like (objects are refcounted in such a way that all reachable objects are kept alive). Also the C library will be calling back into C++ with pointers to this object and the cost of a new wrapper object to be constructed for each C->C++ callback (since C doesn't know about the wrappers) is unacceptable to me.
My general scheme is to do:
class MyType : public mytype {
public:
static MyType* New() { return (MyType*)mytype_new(); }
void DoSomething(int arg) { mytype_dosomething(this, arg); }
};
This will give C++ programmers nicer syntax:
// C Usage:
mytype *t = mytype_new();
mytype_dosomething(t, arg);
// C++ Usage:
MyType *t = MyType::New();
t->DoSomething(arg);
The fib is that I'm downcasting a mytype* (which was allocated with malloc()) to a MyType*, which is a lie. But if MyType has no members and no virtual functions, it seems like I should be able to depend on sizeof(mytype) == sizeof(MyType), and besides MyType has no actual data to which the compiler could generate any kind of reference.
So even though this probably violates the C++ standard, I'm tempted to think that I can get away with this, even across a wide array of compilers and platforms.
My questions are:
Is it possible that, by some streak of luck, this does not actually violate the C++ standard?
Can anyone think of any kind of real-world, practical problem I could run into by using a scheme like this?
EDIT: #James McNellis asks a good question of why I can't define MyType as:
class MyType {
public:
MyType() { mytype_init(this); }
private:
mytype t;
};
The reason is that I have C callbacks that will call back into C++ with a mytype*, and I want to be able convert this directly into a MyType* without having to copy.
You're downcasting a mytype* to a MyType*, which is legal C++. But here it's problematic since the mytype* pointer doesn't actually point to a MyType. It actually points to a mytype. Thus, if you downcast it do a MyType and attempt to access its members, it'll almost certainly not work. Even if there are no data members or virtual functions, you might in the future, and it's still a huge code smell.
Even if it doesn't violate the C++ standard (which I think it does), I would still be a bit suspicious about the code. Typically if you're wrapping a C library the "modern C++ way" is through the RAII idiom:
class MyType
{
public:
// Constructor
MyType() : myType(::mytype_new()) {}
// Destructor
~MyType() { ::my_type_delete(); /* or something similar to this */ }
mytype* GetRawMyType() { return myType; }
const mytype* GetConstRawMyType() const { return myType; }
void DoSomething(int arg) { ::mytype_dosomething(myType, int arg); }
private:
// MyType is not copyable.
MyType(const MyType&);
MyType& operator=(const MyType&);
mytype* myType;
};
// Usage example:
{
MyType t; // constructor called here
t.DoSomething(123);
} // destructor called when scope ends
Is it possible that, by some streak of luck, this does not actually violate the C++ standard?
I'm not advocating this style, but as MyType and mytype are both PODs, I believe the cast does not violate the Standard. I believe MyType and mytype are layout-compatible (2003 version, Section 9.2, clause 14: "Two POD-struct ... types are layout-compatible if they have the same number of nonstatic data members, and corresponding nonstatic data members (in order) have layout-compatible types (3.9)."), and as such can be cast around without trouble.
EDIT: I had to test things, and it turns out I'm wrong. This is not Standard, as the base class makes MyType non-POD. The following doesn't compile:
#include <cstdio>
namespace {
extern "C" struct Foo {
int i;
};
extern "C" int do_foo(Foo* f)
{
return 5 + f->i;
}
struct Bar : Foo {
int foo_it_up()
{
return do_foo(this);
}
};
}
int main()
{
Bar f = { 5 };
std::printf("%d\n", f.foo_it_up());
}
Visual C++ gives the error message that "Types with a base are not aggregate." Since "Types with a base are not aggregate," then the passage I quoted simply doesn't apply.
I believe that you're still safe in that most compilers will make MyType layout-compatible with with mytype. The cast will "work," but it's not Standard.
I think it would be much safer and elegant to have a mytype* data member of MyType, and initialize it in the constructor of MyType rather than having a New() method (which, by the way, has to be static if you do want to have it).
It does violate the c++ standard, however it should work on most (all that I know) compilers .
You're relying on a specific implementation detail here (that the compiler doesn't care what the actual object is, just what is the type you gave it), but I don't think any compiler has a different implementation detail. be sure to check it on every compiler you use, it might catch you unprepared.
From the wikipedia article about Lambda functions and expressions:
users will often wish to define predicate functions near the place
where they make the algorithm function call. The language has only one
mechanism for this: the ability to define a class inside of a
function. ... classes defined in functions do not permit them to be used in templates
Does this mean that use of nested structure inside function is silently deprecated after C++0x lambda are in place ?
Additionally, what is the meaning of last line in above paragraph ? I know that nested classes cannot be template; but that line doesn't mean that.
I'm not sure I understand your confusion, but I'll just state all the facts and let you sort it out. :)
In C++03, this was legal:
#include <iostream>
int main()
{
struct func
{
void operator()(int x) const
{
std::cout << x << std::endl;
}
};
func f; // okay
f(-1); // okay
for (std::size_t i = 0; i < 10; ++i)
f(i) ; // okay
}
But if we tried doing this, it wasn't:
template <typename Func>
void exec(Func f)
{
f(1337);
}
int main()
{
// ...
exec(func); // not okay, local classes not usable as template argument
}
That left us with an issue: we want to define predicates to use for this function, but we can't put it in the function. So we had to move it to whatever outer scope there was and use it there. Not only did that clutters that scope with stuff nobody else needed to know about, but it moved the predicate away from where it's used, making it tougher to read the code.
It could still be useful, for the occasional reused chunk of code within the function (for example, in the loop above; you could have the function predicate to some complex thing with its argument), but most of the time we wanted to use them in templates.
C++0x changes the rules to allow the above code to work. They additionally added lambdas: syntax for creating function objects as expressions, like so:
int main()
{
// same function as above, more succinct
auto func = [](int x){ std::cout << x << std::endl; };
// ...
}
This is exactly like above, but simpler. So do we still have any use for "real" local classes? Sure. Lambda's fall short of full functionality, after all:
#include <iostream>
template <typename Func>
void exec(Func func)
{
func(1337);
}
int main()
{
struct func
{
// note: not possible in C++0x lambdas
void operator()(const char* str) const
{
std::cout << str << std::endl;
}
void operator()(int val) const
{
std::cout << val << std::endl;
}
};
func f; // okay
f("a string, ints next"); // okay
for (std::size_t i = 0; i < 10; ++i)
f(i) ; // okay
exec(f); // okay
}
That said, with lambda's you probably won't see local classes any more than before, but for completely different reasons: one is nearly useless, the other is nearly superseded.
Is there any use case for class inside function after introduction of lambda ?
Definitely. Having a class inside a function is about:
localising it as a private implementation detail of the code intending to use it,
preventing other code using and becoming dependent on it,
being independent of the outer namespace.
Obviously there's a threshold where having a large class inside a function harms readability and obfuscates the flow of the function itself - for most developers and situations, that threshold is very low. With a large class, even though only one function is intended to use it, it may be cleaner to put both into a separate source file. But, it's all just tuning to taste.
You can think of this as the inverse of having private functions in a class: in that situation, the outer API is the class's public interface, with the function kept private. In this situation, the function is using a class as a private implementation detail, and the latter is also kept private. C++ is a multi-paradigm language, and appropriately gives such flexibility in modelling the hierarchy of program organisation and API exposure.
Examples:
a function deals with some external data (think file, network, shared memory...) and wishes to use a class to represent the binary data layout during I/O; it may decide to make that class local if it only has a few fields and is of no use to other functions
a function wants to group a few items and allocate an array of them in support of the internal calculations it does to derive its return value; it may create a simple struct to wrap them up.
a class is given a nasty bitwise enum, or perhaps wants to reinterpret a float or double for access to the mantisa/exponent/sign, and decides internally to model the value using a struct with suitable-width bitfields for convenience (note: implementation defined behaviours)
classes defined in functions do not permit them to be used in templates
I think you commented that someone else's answer had explained this, but anyway...
void f()
{
struct X { };
std::vector<X> xs; // NOPE, X is local
}
Defining structures inside functions was never a particularly good way to deal with the lack of predicates. It works if you have a virtual base, but it's still a pretty ugly way to deal with things. It might look a bit like this:
struct virtual_base {
virtual void operator()() = 0;
};
void foo() {
struct impl : public virtual_base {
void operator()() { /* ... */ }
};
register_callback(new impl);
}
You can still continue to use these classes-inside-functions if you want of course - they're not deprecated or crippled; they were simply restricted from the very start. For example, this code is illegal in versions of C++ prior to C++0x:
void foo() {
struct x { /* ... */ };
std::vector<x> y; // illegal; x is a class defined in a function
boost::function<void()> z = x(); // illegal; x is used to instantiate a templated constructor of boost::function
}
This kind of usage was actually made legal in C++0x, so if anything the usefulness of inner classes has actually be expanded. It's still not really a nice way of doing things most of the time though.
Boost.Variant.
Lambdas don't work with variants, as variants need objects that have more than one operator() (or that have a templated operator()). C++0x allows local classes to be used in templates now, so boost::apply_variant can take them.
As Tony mentioned, a class inside a function is not only about predicates. Besides other use cases, it allows to create a factory function that creates objects confirming to an interface without exposing the implementing class. See this example:
#include <iostream>
/* I think i found this "trick" in [Alexandrescu, Modern C++ Design] */
class MyInterface {
public:
virtual void doSomethingUseful() = 0;
};
MyInterface* factory() {
class HiddenImplementation : public MyInterface {
void doSomethingUseful () {
std::cout << "Hello, World!" << std::endl;
}
};
return new HiddenImplementation();
}
int main () {
auto someInstance = factory();
someInstance->doSomethingUseful();
}
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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.