Converting code from C to C++: typedef void and <unresolved overloaded function type>

Question

I am trying to convert a C file in a C++ file, and I keep having problems with the following definition of typedef. Below I have shown the code and my class structure for A.h, A.cpp and my main class. When I try to compile, I get the following errors:

 main.cpp|44|error: no matching function for call to ‘A::Signal(int, <unresolved overloaded function type>)’|
 main.cpp|44|note: candidate is:|
 A.h|115|note: void (* A::Signal(int, void (*)(int)))(int)|
 A.h|115|note:   no known conversion for argument 2 from ‘<unresolved overloaded function type>’ to ‘void (*)(int)’|

//A.h

class A
{
  public:
  void sigquit_handler (int sig);
  typedef void handler_t(int);
  handler_t *Signal(int signum, handler_t *handler);
}

//A.cpp

/*
 * Signal - wrapper for the sigaction function
 */
 A::handler_t* A::Signal(int signum, A::handler_t *handler) {

   struct sigaction action, old_action;
   action.sa_handler = handler;
   sigemptyset(&action.sa_mask); /* block sigs of type being handled */
   action.sa_flags = SA_RESTART; /* restart syscalls if possible */
   if (sigaction(signum, &action, &old_action) < 0) {
       unix_error("Signal error");
   }

   return (old_action.sa_handler);
 }

/*
 * sigquit_handler - The driver program can gracefully terminate the
 *    child shell by sending it a SIGQUIT signal.
 */
 void A::sigquit_handler(int sig) {
     if (verbose)
        printf("siquit_handler: terminating after SIGQUIT signal\n");
     exit(1);
     }

//main.cpp

int main(int argc, char **argv) {

A a;
a.Signal(SIGQUIT, a.sigquit_handler);  /* so parent can cleanly terminate child*/
}

Can someone explain to me why this is happening? I think that the problem is because of the return type of sigquit_handler(void) and the input parameter of Signal (int, handler_t*) but I can not understand why.

EDITS as suggested:

//A.h

class A
{
  public:
  void sigquit_handler (int sig);
  typedef void (*handler_t)(int);
  handler_t Signal(int,handler_t);
}

//A.cpp

/*
 * Signal - wrapper for the sigaction function
 */
 handler_t A::Signal(int signum, handler_t handler){

   struct sigaction action, old_action;
   action.sa_handler = handler;
   sigemptyset(&action.sa_mask); /* block sigs of type being handled */
   action.sa_flags = SA_RESTART; /* restart syscalls if possible */
   if (sigaction(signum, &action, &old_action) < 0) {
       unix_error("Signal error");
   }

   return (old_action.sa_handler);
 }

/*
 * sigquit_handler - The driver program can gracefully terminate the
 *    child shell by sending it a SIGQUIT signal.
 */
 void A::sigquit_handler(int) {
     if (verbose)
        printf("siquit_handler: terminating after SIGQUIT signal\n");
     exit(1);
     }

//main.cpp

int main(int argc, char **argv) {

A a;
a.Signal(SIGQUIT, &sigquit_handler);  /* so parent can cleanly terminate child*/
}

Error:

 main.cpp|44|error: ‘sigquit_handler’ was not declared in this scope|

Synxis · Accepted Answer · 2013-03-17 18:56:04Z

5

I assume that with this (wrong) line:

typedef void handler_t(int);

You want to make a typedef to a pointer to function. For this, you have to add some parenthesis and a star:

typedef void (*handler_t)(int);

But what you really need is a typedef for a pointer to a member function, which is done like this:

typedef return_type (class_type::* func_t)(arg);

Then (see code comments):

// Second parameter is wrong: you need to use
//     A::handler_t
// instead of:
//     A::handler_t*
//
//      error here                        error here
//          v                                   v
A::handler_t* A::Signal(int signum, A::handler_t* handler)
{
    struct sigaction action, old_action; // 'struct' not needed in C++
    // these 2 variables are not initialized
    // (except if they have a default constructor).

    // ...

    return (old_action.sa_handler); // parenthesis not needed. old_action was not changed in this function
}

Finally, here is how to pass a pointer to member:

A a;
a.Signal(SIGQUIT, &A::sigquit_handler);

Now, another problem: sigaction::sa_handler is not a pointer to member. So, you need to move sigquit_handler out of class A, and change the function Signal:

// A.h
class A
{
    public:
        typedef void (*handler_t)(int);
        handler_t Signal(int,handler_t);
};
void sigquit_handler(int);

// A.cpp
void sigquit_handler(int)
{
    // ...
}

A::handler_t A::Signal(int,handler_t)
{
    // ...
}

// main.cpp
int main()
{
    A a;
    a.Signal(SIGQUIT, &sigquit_handler);
}

edited Mar 17, 2013 at 18:56

answered Mar 17, 2013 at 17:44

Synxis

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12 Comments

FranXh Over a year ago

If I do: typedef void (*handler_t)(int); I get a similar error: A.h|115|note: no known conversion for argument 2 from ‘<unresolved overloaded function type>’ to ‘void (**)(int)’|

Synxis Over a year ago

Yes, see the rest of my answer, you need a typedef to a pointer to member : typedef void (A::* handler_t)(int);

Synxis Over a year ago

See my edit, you had an exta star in one of the parameters of Signal.

Synxis Over a year ago

Edited, you need to use a free function, not a member, for the signal handler.

Synxis Over a year ago

Well, you didn't move sigquit_handler out of the class A. You have to do it.

|

Some programmer dude · Accepted Answer · 2013-03-17 17:46:05Z

3

You have a couple of problems here, one being that you use a pointer to a function, but passes it a member function. Those two are not the same. I suggest you look into std::function and std::bind.

answered Mar 17, 2013 at 17:46

Some programmer dude

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1 Comment

FranXh Over a year ago

for the second problem, my bad, I forgot to add it to the description above; I will edit it now

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