I have not used C++ in a long time. I'm trying to display some polymorphic behavior:
class func {
public:
virtual void print() = 0;
};
class func1 : public func {
public:
void print () { cout << "FUNC 1" << endl; };
};
class func2 : public func {
public:
void print () { cout << "FUNC 2" << endl; };
};
static map<string,func *> myMap;
static func1 f1 = func1 ();
static func2 f2 = func2 ();
myMap["func1"] = &f1;
myMap["func2"] = &f2;
So In my main function, when I call:
myMap["func1"]->print();
myMap["func2"]->print();
I would expect:
FUNC 1
FUNC 2
Not sure if this is the right way to do this. When I compile the code, it gives me this error:
test.cc:31: error: expected constructor, destructor, or type conversion before ‘=’ token
test.cc:32: error: expected constructor, destructor, or type conversion before ‘=’ token
Which refers to these lines:
myMap["func1"] = &f1;
myMap["func2"] = &f2;
Thank you.
Expression statements, like those assignment statements, can only go inside functions.
In C++11, you can initialise the static map using brace-initialisation:
static map<string,func *> myMap = {
{"func1", &f1},
{"func2", &f2}
};
If you're stuck in the past, then either populate it in a function (perhaps main, or something you call before doing anything with the map), or write a function to return a populated map:
std::map<string,func*> make_map() {
std::map<string,func*> map;
map["func1"] = &f1;
map["func2"] = &f2;
return map;
}
static std::map<string,func *> myMap = make_map();
A better idea might be to avoid non-trivial global variables, if possible; they often bring a world of pain.
Related
I have a class with two constructors, one that takes no arguments and one that takes one argument.
Creating objects using the constructor that takes one argument works as expected. However, if I create objects using the constructor that takes no arguments, I get an error.
For instance, if I compile this code (using g++ 4.0.1)...
class Foo
{
public:
Foo() {};
Foo(int a) {};
void bar() {};
};
int main()
{
// this works...
Foo foo1(1);
foo1.bar();
// this does not...
Foo foo2();
foo2.bar();
return 0;
}
... I get the following error:
nonclass.cpp: In function ‘int main(int, const char**)’:
nonclass.cpp:17: error: request for member ‘bar’ in ‘foo2’, which is of non-class type ‘Foo ()()’
Why is this, and how do I make it work?
Foo foo2();
change to
Foo foo2;
You get the error because compiler thinks of
Foo foo2()
as of function declaration with name 'foo2' and the return type 'Foo'.
But in that case If we change to Foo foo2 , the compiler might show the error " call of overloaded ‘Foo()’ is ambiguous".
Just for the record..
It is actually not a solution to your code, but I had the same error message when incorrectly accessing the method of a class instance pointed to by myPointerToClass, e.g.
MyClass* myPointerToClass = new MyClass();
myPointerToClass.aMethodOfThatClass();
where
myPointerToClass->aMethodOfThatClass();
would obviously be correct.
Parenthesis is not required to instantiate a class object when you don't intend to use a parameterised constructor.
Just use Foo foo2;
It will work.
Adding to the knowledge base, I got the same error for
if(class_iter->num == *int_iter)
Even though the IDE gave me the correct members for class_iter. Obviously, the problem is that "anything"::iterator doesn't have a member called num so I need to dereference it. Which doesn't work like this:
if(*class_iter->num == *int_iter)
...apparently. I eventually solved it with this:
if((*class_iter)->num == *int_iter)
I hope this helps someone who runs across this question the way I did.
I was having a similar error, it seems that the compiler misunderstand the call to the constructor without arguments. I made it work by removing the parenthesis from the variable declaration, in your code something like this:
class Foo
{
public:
Foo() {};
Foo(int a) {};
void bar() {};
};
int main()
{
// this works...
Foo foo1(1);
foo1.bar();
// this does not...
Foo foo2; // Without "()"
foo2.bar();
return 0;
}
I ran into a case where I got that error message and had
Foo foo(Bar());
and was basically trying to pass in a temporary Bar object to the Foo constructor. Turns out the compiler was translating this to
Foo foo(Bar(*)());
that is, a function declaration whose name is foo that returns a Foo that takes in an argument -- a function pointer returning a Bar with 0 arguments. When passing in temporaries like this, better to use Bar{} instead of Bar() to eliminate ambiguity.
If you want to declare a new substance with no parameter (knowing that the object have default parameters) don't write
type substance1();
but
type substance;
Certainly a corner case for this error, but I received it in a different situation, when attempting to overload the assignment operator=. It was a bit cryptic IMO (from g++ 8.1.1).
#include <cstdint>
enum DataType
{
DT_INT32,
DT_FLOAT
};
struct PrimitiveData
{
union MyData
{
int32_t i;
float f;
} data;
enum DataType dt;
template<typename T>
void operator=(T data)
{
switch(dt)
{
case DT_INT32:
{
data.i = data;
break;
}
case DT_FLOAT:
{
data.f = data;
break;
}
default:
{
break;
}
}
}
};
int main()
{
struct PrimitiveData pd;
pd.dt = DT_FLOAT;
pd = 3.4f;
return 0;
}
I received 2 "identical" errors
error: request for member ‘i’ [and 'f'] in ‘data’, which is of non-class type ‘float’
(The equivalent error for clang is:
error: member reference base type 'float' is not a structure or union)
for the lines data.i = data; and data.f = data;. Turns out the compiler was confusing local variable name 'data' and my member variable data. When I changed this to void operator=(T newData) and data.i = newData;, data.f = newData;, the error went away.
#MykolaGolubyev has already given wonderful explanation. I was looking for a solution to do somthing like this MyClass obj ( MyAnotherClass() ) but the compiler was interpreting it as a function declaration.
C++11 has braced-init-list. Using this we can do something like this
Temp t{String()};
However, this:
Temp t(String());
throws compilation error as it considers t as of type Temp(String (*)()).
#include <iostream>
class String {
public:
String(const char* str): ptr(str)
{
std::cout << "Constructor: " << str << std::endl;
}
String(void): ptr(nullptr)
{
std::cout << "Constructor" << std::endl;
}
virtual ~String(void)
{
std::cout << "Destructor" << std::endl;
}
private:
const char *ptr;
};
class Temp {
public:
Temp(String in): str(in)
{
std::cout << "Temp Constructor" << std::endl;
}
Temp(): str(String("hello"))
{
std::cout << "Temp Constructor: 2" << std::endl;
}
virtual ~Temp(void)
{
std::cout << "Temp Destructor" << std::endl;
}
virtual String get_str()
{
return str;
}
private:
String str;
};
int main(void)
{
Temp t{String()}; // Compiles Success!
// Temp t(String()); // Doesn't compile. Considers "t" as of type: Temp(String (*)())
t.get_str(); // dummy statement just to check if we are able to access the member
return 0;
}
I've inherited some C++ code and I've been tasked with getting rid of warnings.
Here we have a member function pointer being cast to a function pointer.
I understand that member function pointers are "different" from function pointers, in that there is an implicit 'this' parameter involved under the hood. However my predecessor appears to have made explicit use of this fact, by casting from a member function pointer to a function pointer with an additional first parameter inserted.
My Questions are:
A) Can I get rid of the compiler warning?
B) To what extent is this code guaranteed to work?
I've cut it down to a small main.cpp for the purposes of this question:
#define GENERIC_FUNC_TYPE void(*)(void)
#define FUNC_TYPE int(*)(void *)
class MyClass
{
public:
MyClass(int a) : memberA(a) {}
int myMemberFunc()
{
return memberA;
}
private:
int memberA;
};
int main(int argc, char*argv[])
{
int (MyClass::* memberFunc) () = &MyClass::myMemberFunc;
MyClass myObject(1);
std::cout << (myObject.*memberFunc)() << std::endl;
// All good so far
// Now get naughty, store it away in a very basic fn ptr
void(*myStoredFunction)(void) = (GENERIC_FUNC_TYPE)memberFunc; // Compiler warning
// Reinterpret the fn pointer as a pointer to fn, with an extra object parameter
int (*myExtractedFunction)(void*) = (FUNC_TYPE)myStoredFunction;
// Call it
std::cout << myExtractedFunction(&myObject) << std::endl;
}
The code compiles with one warning under g++, and as intended outputs two 1's:
main.cpp: In function ‘int main(int, char**)’:
main.cpp:27:53: warning: converting from ‘int (MyClass::*)()’ to ‘void (*)()’ [-Wpmf-conversions]
void(*myStoredFunction)(void) = (GENERIC_FUNC_TYPE)memberFunc; // Compiler warning
^
IMHO this code is making assumptions about the underlying mechanisms of the compiler. Or maybe these assumptions are valid for all C++ compilers - Can anyone help?
(In the actual code we're storing a whole bunch of function pointers by name in a map. These functions all have different signatures, which is why they are all cast to the same signature void(*)(void). This is analogous to the myStoredFunction above. They are then cast to the individual signatures at the point of calling, analogous to myExtractedFunction above.)
How about create functions which avoid the cast entirely:
template <typename C, void (C::*M)()>
void AsFunc(void* p)
{
(static_cast<C*>(p)->*M)();
}
then
void (*myExtractedFunction)(void*) = &AsFunc<MyClass, &MyClass::myMemberFunc>;
In C++17, with some traits, you might even have template <auto *M> void AsFunc(void* p) and void(*myStoredFunction)(void*) = &AsFunc<&MyClass::myMemberFunc>;
To answer the question in the title, no, you can't legally cast a pointer-to-member-function to a pointer-to-function. Presumably, that's what the "Compiler warning" on the line with that cast said.
A conforming compiler is required to issue a diagnostic when confronted with ill-formed code (that's a bit oversimplified), and this one did. It gave a warning. Having done that, the compiler is free to do something implementation-specific, which it seems to have done: it compiled the code into something that does what you were hoping for.
Compilers are free to represent pointers to member functions in any way that works, and for non-virtual functions, that could be just a "normal" pointer to function. But try that with a virtual function; I'll bet the consequences are more harsh.
A) Can I get rid of the compiler warning?
Yes - wrap the member function in a call from a static function
(This is a low-tech variant of #Jarod42's template based answer)
B) To what extent is this code guaranteed to work?
It's not (summarizing #Pete Becker's answer). Until you get rid of the warning.
Here's the jist of what we went with. We kept it simple to minimize disruption to the code. We avoided advanced C++ features to maximize the number of people who can work on the code.
#include <iostream>
class MyClass
{
public:
MyClass(int a) : memberA(a) {}
static int myMemberFuncStatic(MyClass *obj)
{
return obj->myMemberFunc();
}
int myMemberFunc()
{
return memberA;
}
private:
int memberA;
};
typedef void(*GENERIC_FUNC_TYPE)(void);
typedef int(*FUNC_TYPE)(MyClass *);
int main(int argc, char*argv[])
{
int (* staticFunc) (MyClass *) = &MyClass::myMemberFuncStatic;
MyClass myObject(1);
std::cout << staticFunc(&myObject) << std::endl;
// All good so far
// This is actually legal, for non-member functions (like static functions)
GENERIC_FUNC_TYPE myStoredFunction = reinterpret_cast<GENERIC_FUNC_TYPE> (staticFunc); // No compiler warning
// Reinterpret the fn pointer as the static function
int (*myExtractedFunction)(MyClass*) = (FUNC_TYPE)myStoredFunction;
// Call it
std::cout << myExtractedFunction(&myObject) << std::endl;
}
Since you apparently need to call a function by name on some "untyped" object (void*) while passing in a number of arguments that differ by function, you need some kind of multiple-dispatch. A possible solution is:
#include <string>
#include <iostream>
#include <stdexcept>
#include <functional>
#include <utility>
#include <map>
template <typename Subj>
using FunctionMap = std::map<std::string, std::function<void (Subj&, const std::string&)>>;
class AbstractBaseSubject {
public:
virtual void invoke (const std::string& fName, const std::string& arg) = 0;
};
template <typename Class>
class BaseSubject : public AbstractBaseSubject {
public:
virtual void invoke (const std::string& fName, const std::string& arg) {
const FunctionMap<Class>& m = Class::functionMap;
auto iter = m.find (fName);
if (iter == m.end ())
throw std::invalid_argument ("Unknown function \"" + fName + "\"");
iter->second (*static_cast<Class*> (this), arg);
}
};
class Cat : public BaseSubject<Cat> {
public:
Cat (const std::string& name) : name(name) {}
void meow (const std::string& arg) {
std::cout << "Cat(" << name << "): meow (" << arg << ")\n";
}
static const FunctionMap<Cat> functionMap;
private:
std::string name;
};
const FunctionMap<Cat> Cat::functionMap = {
{ "meow", [] (Cat& cat, const std::string& arg) { cat.meow (arg); } }
};
class Dog : public BaseSubject<Dog> {
public:
Dog (int age) : age(age) {}
void bark (float arg) {
std::cout << "Dog(" << age << "): bark (" << arg << ")\n";
}
static const FunctionMap<Dog> functionMap;
private:
int age;
};
const FunctionMap<Dog> Dog::functionMap = {
{ "bark", [] (Dog& dog, const std::string& arg) { dog.bark (std::stof (arg)); }}
};
int main () {
Cat cat ("Mr. Snuggles");
Dog dog (7);
AbstractBaseSubject& abstractDog = dog; // Just to demonstrate that the calls work from the base class.
AbstractBaseSubject& abstractCat = cat;
abstractCat.invoke ("meow", "Please feed me");
abstractDog.invoke ("bark", "3.14");
try {
abstractCat.invoke ("bark", "3.14");
} catch (const std::invalid_argument& ex) {
std::cerr << ex.what () << std::endl;
}
try {
abstractCat.invoke ("quack", "3.14");
} catch (const std::invalid_argument& ex) {
std::cerr << ex.what () << std::endl;
}
try {
abstractDog.invoke ("bark", "This is not a number");
} catch (const std::invalid_argument& ex) {
std::cerr << ex.what () << std::endl;
}
}
Here, all classes with functions to be called this way need to derive from BaseSubject (which is a CRTP). These classes (here: Cat and Dog, let's call them "subjects") have different functions with different arguments (bark and meow - of course more than one function per subject is possible). Each subject has its own map of string-to-function. These functions are not function pointers, but std::function<void (SubjectType&,const std::string&)> instances. Each of those should call the respective member function of the object, passing in the needed arguments. The arguments need to come from some kind of generic data representation - here, I chose a simple std::string. It might be a JSON or XML object depending on where your data comes from. The std::function instances need to deserialize the data and pass it as arguments. The map is created as a static variable in each subject class, where the std::function instances are populated with lambdas. The BaseSubject class looks up the function instance and calls it. Since the subject class should always directly derive from BaseSubject<Subject>, pointers of type BaseSubject<Subject>* may be directly and safely cast to Subject*.
Note that there is no unsafe cast at all - it is all handled by virtual functions. Therefore, this should be perfectly portable. Having one map per subject class is typing-intensive, but allows you to have identically-named functions in different classes. Since some kind of data-unpacking for each function individually is necessary anyways, we have individual unpacking-lambdas inside the map.
If a function's arguments are just the abstract data structure, i.e. const std::string&, we could leave the lambdas out and just do:
const FunctionMap<Cat> Cat::functionMap = {
{ "meow", &Cat::meow }
};
Which works by way of std::functions magic (passing this via the 1st argument), which, in contrast to function pointers, is well-defined and allowed. This would be particularly useful if all functions have the same signature. In fact, we could then even leave out the std::function and plug in Jarod42's suggestion.
PS: Just for fun, here's an example where casting a member-function-pointer to an function-pointer fails:
#include <iostream>
struct A {
char x;
A () : x('A') {}
void foo () {
std::cout << "A::foo() x=" << x << std::endl;
}
};
struct B {
char x;
B () : x('B') {}
void foo () {
std::cout << "B::foo() x=" << x << std::endl;
}
};
struct X : A, B {
};
int main () {
void (B::*memPtr) () = &B::foo;
void (*funPtr) (X*) = reinterpret_cast<void (*)(X*)> (memPtr); // Illegal!
X x;
(x.*memPtr) ();
funPtr (&x);
}
On my machine, this prints:
B::foo() x=B
B::foo() x=A
The B class shouldn't be able to print "x=A"! This happens because member-function pointers carry an extra offset that is added to this before the call, in case multiple inheritance comes into play. Casting loses this offset. So, when calling the casted function pointer, this automatically refers to the first base object, while B is the second, printing the wrong value.
PPS: For even more fun:
If we plug in Jarod42's suggestion:
template <typename C, void (C::*M)(), typename Obj>
void AsFunc (Obj* p) {
(p->*M)();
}
int main () {
void (*funPtr) (X*) = AsFunc<B, &B::foo, X>;
X x;
funPtr (&x);
}
the program correctly prints:
B::foo() x=B
If we look at the disassembly of AsFunc, we see:
c90 <void AsFunc<B, &B::foo, X>(X*)>:
c90: 48 83 c7 01 add $0x1,%rdi
c94: e9 07 ff ff ff jmpq ba0 <B::foo()>
The compiler automatically generated code that adds 1 to the this pointer, such that B::foo is called with this pointing to the B base class of X. To make this happen in the AsFunc function (opposed to buried within main), I introduced the Obj template parameter which lets the p argument be of the derived type X such that AsFunc has to do the adding.
I have a class with two constructors, one that takes no arguments and one that takes one argument.
Creating objects using the constructor that takes one argument works as expected. However, if I create objects using the constructor that takes no arguments, I get an error.
For instance, if I compile this code (using g++ 4.0.1)...
class Foo
{
public:
Foo() {};
Foo(int a) {};
void bar() {};
};
int main()
{
// this works...
Foo foo1(1);
foo1.bar();
// this does not...
Foo foo2();
foo2.bar();
return 0;
}
... I get the following error:
nonclass.cpp: In function ‘int main(int, const char**)’:
nonclass.cpp:17: error: request for member ‘bar’ in ‘foo2’, which is of non-class type ‘Foo ()()’
Why is this, and how do I make it work?
Foo foo2();
change to
Foo foo2;
You get the error because compiler thinks of
Foo foo2()
as of function declaration with name 'foo2' and the return type 'Foo'.
But in that case If we change to Foo foo2 , the compiler might show the error " call of overloaded ‘Foo()’ is ambiguous".
Just for the record..
It is actually not a solution to your code, but I had the same error message when incorrectly accessing the method of a class instance pointed to by myPointerToClass, e.g.
MyClass* myPointerToClass = new MyClass();
myPointerToClass.aMethodOfThatClass();
where
myPointerToClass->aMethodOfThatClass();
would obviously be correct.
Parenthesis is not required to instantiate a class object when you don't intend to use a parameterised constructor.
Just use Foo foo2;
It will work.
Adding to the knowledge base, I got the same error for
if(class_iter->num == *int_iter)
Even though the IDE gave me the correct members for class_iter. Obviously, the problem is that "anything"::iterator doesn't have a member called num so I need to dereference it. Which doesn't work like this:
if(*class_iter->num == *int_iter)
...apparently. I eventually solved it with this:
if((*class_iter)->num == *int_iter)
I hope this helps someone who runs across this question the way I did.
I was having a similar error, it seems that the compiler misunderstand the call to the constructor without arguments. I made it work by removing the parenthesis from the variable declaration, in your code something like this:
class Foo
{
public:
Foo() {};
Foo(int a) {};
void bar() {};
};
int main()
{
// this works...
Foo foo1(1);
foo1.bar();
// this does not...
Foo foo2; // Without "()"
foo2.bar();
return 0;
}
I ran into a case where I got that error message and had
Foo foo(Bar());
and was basically trying to pass in a temporary Bar object to the Foo constructor. Turns out the compiler was translating this to
Foo foo(Bar(*)());
that is, a function declaration whose name is foo that returns a Foo that takes in an argument -- a function pointer returning a Bar with 0 arguments. When passing in temporaries like this, better to use Bar{} instead of Bar() to eliminate ambiguity.
If you want to declare a new substance with no parameter (knowing that the object have default parameters) don't write
type substance1();
but
type substance;
Certainly a corner case for this error, but I received it in a different situation, when attempting to overload the assignment operator=. It was a bit cryptic IMO (from g++ 8.1.1).
#include <cstdint>
enum DataType
{
DT_INT32,
DT_FLOAT
};
struct PrimitiveData
{
union MyData
{
int32_t i;
float f;
} data;
enum DataType dt;
template<typename T>
void operator=(T data)
{
switch(dt)
{
case DT_INT32:
{
data.i = data;
break;
}
case DT_FLOAT:
{
data.f = data;
break;
}
default:
{
break;
}
}
}
};
int main()
{
struct PrimitiveData pd;
pd.dt = DT_FLOAT;
pd = 3.4f;
return 0;
}
I received 2 "identical" errors
error: request for member ‘i’ [and 'f'] in ‘data’, which is of non-class type ‘float’
(The equivalent error for clang is:
error: member reference base type 'float' is not a structure or union)
for the lines data.i = data; and data.f = data;. Turns out the compiler was confusing local variable name 'data' and my member variable data. When I changed this to void operator=(T newData) and data.i = newData;, data.f = newData;, the error went away.
#MykolaGolubyev has already given wonderful explanation. I was looking for a solution to do somthing like this MyClass obj ( MyAnotherClass() ) but the compiler was interpreting it as a function declaration.
C++11 has braced-init-list. Using this we can do something like this
Temp t{String()};
However, this:
Temp t(String());
throws compilation error as it considers t as of type Temp(String (*)()).
#include <iostream>
class String {
public:
String(const char* str): ptr(str)
{
std::cout << "Constructor: " << str << std::endl;
}
String(void): ptr(nullptr)
{
std::cout << "Constructor" << std::endl;
}
virtual ~String(void)
{
std::cout << "Destructor" << std::endl;
}
private:
const char *ptr;
};
class Temp {
public:
Temp(String in): str(in)
{
std::cout << "Temp Constructor" << std::endl;
}
Temp(): str(String("hello"))
{
std::cout << "Temp Constructor: 2" << std::endl;
}
virtual ~Temp(void)
{
std::cout << "Temp Destructor" << std::endl;
}
virtual String get_str()
{
return str;
}
private:
String str;
};
int main(void)
{
Temp t{String()}; // Compiles Success!
// Temp t(String()); // Doesn't compile. Considers "t" as of type: Temp(String (*)())
t.get_str(); // dummy statement just to check if we are able to access the member
return 0;
}
Consider the following class structure:-
class foo {
public:
int fun () {
cout << "in foo" << endl;
}
};
class bar_class1:public foo {
public:
int fun () {
cout << "in bar_class1" << endl;
}
};
class bar_class2:public foo {
public:
float fun () {
cout << "in bar_class2" << endl;
}
};
main () {
foo * foo_pointer = new bar_class1();
foo_pointer->fun();
}
The output of the above program is in foo. Is there a way, that using a pointer of type foo * which actually points to an object of type bar_class1 or bar_class2, we can call the fun function of the derived class instead of the base class? I am not able to make the fun function virtual in the base class foo since, then there is a return type conflict for function foo in the derived class bar_class2.
Here's my comments as an answer.
You cannot do that.
If that kind of polymorphism were possible, wouldn't that break horribly when code calls foo::fun (expecting an int) on an object whose actual type is bar_class2 and thus gets a float? Do you want to simply throw away type safety?
If you want different return types, sounds like you want a template. But you cannot use templates quite in the way that you want to use foo(). Static polymorphism (templates) and run time polymorphism (late binding) don't mix well. You need to redesign your oop structure.
If you absolutely hate type safety, you can sort of do this with void pointers. But for the love of Flying Spaghetti Monster, don't ever do this in c++. Please close your eyes before reading the following code to avoid exposure.
#include <iostream>
class foo {
public:
virtual void* fun() = 0;
virtual ~foo(){};
};
class bar_class1: public foo {
public:
void* fun() {
return &value;
}
private:
int value = 1;
};
class bar_class2: public foo {
public:
void* fun() {
return &value;
}
private:
float value = 1.1;
};
int main() {
foo* foo_pointer1 = new bar_class1();
foo* foo_pointer2 = new bar_class2();
// in c++ compiler must know the type of all objects during compilation
std::cout << *reinterpret_cast<int*>(foo_pointer1->fun()) << '\n';
std::cout << *reinterpret_cast<float*>(foo_pointer2->fun()) << '\n';
delete foo_pointer1;
delete foo_pointer2;
}
Perhaps similar to the existing answer, I really hope you realize changing your design is better than this mess, but I believe it's the best you're going to get. I force you to specify the return type at the callsite (e.g., someFoo->fun<int>()), since you're going to have to know it anyway, and dispatch based on that. Any funny business and you'll get an exception. Also keep in mind the performance of this is, I imagine, less than desirable.
#include <cassert>
#include <stdexcept>
#include <type_traits>
struct foo {
virtual ~foo() = default;
template<typename T, typename = typename std::enable_if<std::is_same<T, int>::value>::type, typename = void>
T fun();
template<typename T, typename = typename std::enable_if<std::is_same<T, float>::value>::type>
T fun();
};
struct bar_class1 : foo {
int fun() {
return 2;
}
};
struct bar_class2 : foo {
float fun() {
return 3.5f;
}
};
template<typename T, typename, typename Dummy>
T foo::fun() {
if (auto *p = dynamic_cast<bar_class1 *>(this)) {
return p->fun();
} else if (dynamic_cast<bar_class2 *>(this)) {
throw std::invalid_argument("Mismatching dynamic type.");
} else {
return 1;
}
}
template<typename T, typename>
T foo::fun() {
auto *p = dynamic_cast<bar_class2 *>(this);
if (dynamic_cast<bar_class1 *>(this) || !p) {
throw std::invalid_argument("Mismatching dynamic type.");
} else if (auto *p = dynamic_cast<bar_class2 *>(this)) {
return p->fun();
}
assert(false); //should never get here, but compiler doesn't know that
}
If you'd like the main function, I've written a complete sample.
To answer your question: No, late binding isn't possible without deciding the return type. ...at least not in a reasonable manner, see user2079303's great counter-example. But...
you may change your code (for example) into something like the following, using the keyword virtual and equalize the return type for instance to void:
class foo
{
public:
virtual void fun(std::ostream& out) {
out << "in foo" << std::endl;
}
};
so you can decide the output type later:
class intFoo: public foo
{
public:
void fun(std::ostream& out) {
// output an int
out << "in bar_class1. data: " << data << endl;
}
int data;
};
class floatFoo: public foo
{
public:
void fun(std::ostream& out) {
// output a float
out << "in bar_class2. data: " << data << endl;
}
float data;
};
For brevity, I double-use the output stream - now a parameter of the function fun() - function to demonstrate type-dependent portion of your derived class. In your application, the parameter will probably be of another, more useful type.
The function fun is not a virtual function since you didn't use the keyword "virtual" to decorate it. So, the compile will determine which function to call at compiling time. So, there is no way to tell the compiler to call another function because the compiler will use its static type, i.e. the variable definition type -- foo *.
I want to define a member function in class and use its pointer. I know that I can use static member function but the problem with it is that I can only access the static members of the class. Is there a way other than static member function to be able to get function pointer.
To be more specific: There is a library which I'm using which gets a function pointer as its input. I want to write a member function and assign its function pointer to that external library. Should I create an object of class or use this pointer to do this?
You can get the pointer of the method, but it has to be called with an object
typedef void (T::*MethodPtr) ();
MethodPtr method = &T::MethodA;
T *obj = new T();
obj->*method();
If you need to have non-object pointer and you want to use object then you have to store instance of object somewhere, but you are restricted to use only one object (singleton).
class T {
static T *instance;
public:
T::T() {
instance = this;
}
static void func() {
instance->doStuff();
}
void doStuff() {}
};
If library supports user data for function pointers, then you may have multiple instances
class T {
public:
static void func(void *instance) {
((T*)instance)->doStuff();
}
void doStuff() {}
};
If:
you want to get the function pointer of a nonstatic member from within the class
And use it within the class:
Then:
It can work, because when you get the member function address, there is a "this" pointer. The syntax was not obvious to me, and it may look somewhat ugly, but not TOO bad.
This may not be new to the true experts, but I have wanted to have this in my bag of tricks for a long time.
Here is a complete sample program:
#include <iostream>
class CTestFncPtr
{
public:
CTestFncPtr(int data) : mData(data)
{
// Switch = &CTestFncPtr::SwitchC; // Won't compile - wrong function prototype - this is type safe
if (data == 1)
Switch = &CTestFncPtr::SwitchA;
else
Switch = &CTestFncPtr::SwitchB;
}
void CallSwitch(char *charData)
{
(this->*Switch)(charData);
}
private:
void SwitchA(char * charData)
{
std::cout << "Called Switch A " << "Class Data is " << mData<<" Parameter is " << charData<< "\n";
Switch = &CTestFncPtr::SwitchB;
}
void SwitchB(char * charData)
{
std::cout << "Called Switch B " << "Class Data is " << mData<<" Parameter is " << charData<< "\n";
Switch = &CTestFncPtr::SwitchA;
}
void SwitchC()
{
}
void(CTestFncPtr::*Switch)(char * charData);
int mData;
};
int main(int argc, char * argv[])
{
CTestFncPtr item1(1);
item1.CallSwitch("Item1");
item1.CallSwitch("Switched call Item 1");
CTestFncPtr item2(0);
item2.CallSwitch("Item2");
item2.CallSwitch("Switched call Item 2");
return 0;
}