funtion pointer

Natural_Bugger

hi there,

i'm trying to relearn function pointers "again" and i need some help.
this is my class:

class test
{
public:
    test();
    void test2();
    ~test();

    int origin;

    int * number;
    void (test:: *foo)(void);

};

void test::test2(){
    std::cout << "hi" <<std::endl;
}

this is my Main():

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    test first;

    first.foo = &test::test2; // compiles
   //first.foo = &first.test2; // does not complile

    first.foo(); // <-- i wanna call my newly configured function pointer.

    return a.exec();
}

these are the errors:

main.cpp:19: error: called object type 'void (test::*)()' is not a function or function pointer
main.cpp:19: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘first.test::foo (...)’, e.g. ‘(... ->* first.test::foo) (...)’
   19 |     first.foo();
      |               ^

kind regards

why?

void (test:: *foo)(void);

and not

static void (*foo)(void);

[Moved to C++ Gurus ~kshegunov]

kshegunov

@Natural_Bugger said in funtion pointer:

int main(int argc, char *argv[])
{
    // ...
    test first;
    first.foo = &test::test2; //< Assigning an address to a pointer is correct, so it compiles
    // first.foo = &first.test2;  //< Makes no sense `test2` isn't a member variable or a function call, so it can't be accessed with `.`

    (first.*(first.foo))(); //< Call a method through a PMF
    // ...
}

these are the errors:

main.cpp:19: error: called object type 'void (test::*)()' is not a function or function pointer
main.cpp:19: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘first.test::foo (...)’, e.g. ‘(... ->* first.test::foo) (...)’
   19 |     first.foo();
      |               ^

See the comments above.

why?

void (test:: *foo)(void);

and not

static void (*foo)(void);

Static functions are no different from global functions, with the exception of their scope. So a static function has the same prototype as a global, that is:

class A
{
    static void x();
};

void y();

decltype(&A::x) //< void (*)();
decltype(y) //< void (*)();

Methods on the other hand need to have an object to be called upon, hence they're bound to a specific instance of a specific class. Thus the prototype of a method must specify also the class it belongs to (low-level details are unimportant here):

class A
{
    void z();
};

decltype(&A::z) //< void (A::*)();

Natural_Bugger

@kshegunov

Thnx a lot, for your comprehensive reply.

how about if you create the function pointer locally and point to a member of a class.
if i stated that correctly.

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

     test first;

    void (*foo)(void);
    foo = &test::test2;


    return a.exec();
}

returns:

main.cpp:22: error: assigning to 'void ()()' from incompatible type 'void (test::)()'
main.cpp:22: error: cannot convert ‘void (test::)()’ to ‘void ()()’ in assignment
22 | foo = &test::test2;
| ^~~~~

Natural_Bugger

@kshegunov

now i'm trying to create a function pointer that takes an argument and returns a value.

test.h:

class test
{
public:
    test();

    void test2();
    int test3(int a);

    ~test();

    void (test:: *foo)(void);
    int (test:: *foo2)(int);
    int (test:: *foo3)();

};

#endif // TEST_H

test.cpp:

.
..
...
void test::test2(){
    std::cout << "hi" <<std::endl;
}

int test3(int a){
return a;
}

int test4(){
return 10;
}

main:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    test first;

    first.foo = &test::test2;
    (first.*(first.foo))();

    first.foo2 = &test::test3; // <--- error
    (first.*(first.foo2))(10);

   first.foo3 = &test::test4; <--- error
    std::cout << "Foo: " + std::to_string((first.*(first.foo3))) <<std::endl; <---error

    return a.exec();
}

error:

main.cpp:27: error: undefined reference to `test::test3(int)'

first.foo3 = &test::test4; // <--- error, with the next line commented.
//std::cout << "Foo: " + std::to_string((first.*(first.foo3))) <<std::endl;

// uncommented, it doesn't show an error on that line, but on line:
// std::cout << "Foo: " + std::to_string((first.*(first.foo3))) <<std::endl;
main.cpp:31: error: invalid use of non-static member function of type ‘int (test::)()’
   31 |     std::cout << "Foo: " + std::to_string((first.*(first.foo3))) <<std::endl;
      |                                           ~~~~~~^~~~~~~~~~~~~~~
main.cpp:31: error: reference to non-static member function must be called; did you mean to call it with no arguments?

i tried:

first.foo2 = &test::test3(int);

but results is more errors:

main.cpp:27: error: call to non-static member function without an object argument
main.cpp:27: error: expected '(' for function-style cast or type construction
main.cpp:27: error: expected primary-expression before ‘int’
   27 |     first.foo2 = &test::test3(int);
      |                               ^~~

kind regards

jsulm

@Natural_Bugger said in funtion pointer:

int test3(int a){

int test::test3(int a){

Natural_Bugger

@jsulm

ay, i didn't notice..
thanks alot.