C++ calling a function pointer stored in a struct in a map

Question

I am getting the error term does not evaluate to a function taking 1 arguments when trying to call a function pointer.

The function pointer is stored in a struct. The struct is then stored in a map. Definition:

typedef void (CLIOptions::*OptionHandler)(QString);

struct OptionDefinition {
   QString name;
   QString description;
   QString type;
   OptionHandler handler;
};

typedef std::map<QString, OptionDefinition> CLIOptionMap;

I initialise the map like this:

CLIOptionMap optionMap =
{
   {
      QString("set-tcp-host"),
      {
         QString("set-tcph"),
         QString("Set the TCP server host address"),
         QString("string"),
         &CLIOptions::setTcpHost
      }
   },
  // etc...
}

The problem occurs when I try to iterate through the map and call the handler:

for (it = optionMap.begin(); it != optionMap.end(); ++it) {
    QString value = /*snip...*/

    (it->second.handler)(value)
}

What is my problem here?

How would the line of code you wrote know what instance of CLIOptions to call that member function on? — Mat, Jan 05 '18 at 09:57
duplicate of https://stackoverflow.com/questions/1485983/calling-c-class-methods-via-a-function-pointer me thinks — UKMonkey, Jan 05 '18 at 09:57
@EdgarRokyan : No. `it->second.handler` is the pmf. You then need something like `this->*(it->second.handler)(value)`. This is complicated enough, I recommend an intermediate variable for readability. — Martin Bonner supports Monica, Jan 05 '18 at 10:01

Martin Bonner supports Monica · Accepted Answer · 2018-01-05T10:26:55.283

3

Your problem is that you don't have a function pointer, you have a pointer to member function, and they are very different beasts. A pointer-to-member-function isn't even a pointer in general (it has to be able to handle pointer to a virtual function in a second virtual base class!)

Given you have a pmf, you need an object to invoke the pmf on. So something like:

for (it = optionMap.begin(); it != optionMap.end(); ++it) {
    QString value = /*snip...*/
    const auto pmf = it->second.handler;
    (mOptionHandler.*pmf)(value);
}

actually, if you going to use C++11 auto, you can also use the foreach loop:

for (const auto& option : optionMap) {
   const auto pmf = option.handler;
   (mOptionHandler.*pmf)(option.value);
}

edited Jan 05 '18 at 10:26

answered Jan 05 '18 at 09:57

Martin Bonner supports Monica

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Thanks. So I have an instance of "CLIOptions" available called ``mOptionHandler``, so I setup the call like ``(mOptionHandler.pmf)`` but this gives ``CLIOptions has no member pmf`` – jramm Jan 05 '18 at 10:06
Oh I just missed a dereference I think: ``mOptionHandler.*pmf`` – jramm Jan 05 '18 at 10:09
I've changed my example to use your name. And yes - the `.*` operator is completely different to the `.` operator! – Martin Bonner supports Monica Jan 05 '18 at 10:27

C++ calling a function pointer stored in a struct in a map

1 Answers1