c++ calling function when using map of function pointer

Question

I set up this account mainly because I couldn't find answers elsewhere. I checked, various tutorials or questions/ answers on stackoverflow and different pages.

I'm programming a terminal based textadventure and need a map of functions. This is what I got (I left out all the stuff that isn't of interesting for the problem)

#include <map>

using namespace std;

class CPlayer
{
private:

    //Players functions:
    typedef void(CPlayer::*m_PlayerFunction)(void); //Function-pointer points to various player 
                                                    //functions
    map<char*, m_PlayerFunction> *m_FunctionMap;    //Map containing all player functions

public:
    //Constructor
    CPlayer(char* chName, CRoom* curRoom, CInventory* Inventory);


    //Functions:
    bool useFunction(char* chPlayerCommand);
    void showDoors(); //Function displaing all doors in the room
    void showPeople(); //Function displaying all people in the room


};

#endif

#include "CPlayer.h"
#include <iostream>


CPlayer::CPlayer(char chName[128], CRoom* curRoom, CInventory *Inventory)
{
    //Players functions
    m_FunctionMap = new map<char*, CPlayer::m_PlayerFunction>;
    m_FunctionMap->insert(std::make_pair((char*)"show doors", &CPlayer::showDoors));
    m_FunctionMap->insert(std::make_pair((char*)"show people", &CPlayer::showPeople));
}






//Functions

//useFunction, calls fitting function, return "false", when no function ist found
bool CPlayer::useFunction(char* chPlayerCommand)
{
    CFunctions F;
    map<char*, m_PlayerFunction>::iterator it = m_FunctionMap->begin();

    for(it; it!=m_FunctionMap->end(); it++)
    {
        if(F.compare(chPlayerCommand, it->first) == true)
        {
            cout << "Hallo" << endl;
            (it->*second)();
        }
    }

    return false;
}

Now, the problem is the following:

If I call the function this way: (it->*second)(); which seems to be how it is supposed to be done, I get the following error: error: ‘second’ was not declared in this scope

If I call the function this way: (*it->second)(); which is what I got from this thread: Using a STL map of function pointers, I get following error: error: invalid use of unary ‘ * ’ on pointer to member

I'd be very glad if someone could help me. Thanks ahead for all the upcoming answers.

PS: It would also be interesting to know whether "map" or "unordered_map" is the better way of solving this problem.

As I said, thanks ahead: GB

Possible duplicate of [How do I call a member function pointer?](https://stackoverflow.com/questions/24325612/how-do-i-call-a-member-function-pointer) — alain, Jun 21 '17 at 14:27
Did the proposed duplicate question or searching for "calling pointer to member function" help you solve the problem? — nwp, Jun 21 '17 at 14:37
There is issue with using `const char *` as key in a map. And there is no reason to make it worse casting it to `char *` — Slava, Jun 21 '17 at 14:42
Using pointers as map keys is very unlikely to work well. And iterating over a map looking for a key is not the way to use a map. — molbdnilo, Jun 21 '17 at 14:58
I don't think that's an exact duplicate of the above question. There are two operators `->*` and `.*`, and these two questions cover each one of them. — Ghislain Fourny, Jun 21 '17 at 14:58
You need a `std::unordered_map`, and hold it by value! That way you can just do `auto it = m_FunctionMap.find(chPlayerCommand); if (it != m_FunctionMap.end()) (this->*(it->second))();` — Martin Bonner supports Monica, Jun 21 '17 at 16:22
Reason I'm using char*, is that the player Command might also contain people, items, door or room names, which are also char*, as they might change during the game, thus char* not const char. — georgbüchner, Jun 22 '17 at 07:50

Ghislain Fourny · Accepted Answer · 2017-06-21T14:54:53.340

2

The difficulty may be that it is at the same time a map, and it involves pointer-to-members, which makes the syntax to invoke more complicated with lots of parentheses that must be at the right positions. I think it should be something like this:

(this->*(it->second))()

Alternatively, as Rakete1111 points out, the following works as well:

(this->*it->second)()

(Note that the latter is less verbose, but also less easy to read for people who do not have the operator precedence on the top of their minds).

edited Jun 21 '17 at 14:54

answered Jun 21 '17 at 14:40

Ghislain Fourny

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Yes, but you don't need the nested ones: `(this->*it->second)()` is ok too. – Rakete1111 Jun 21 '17 at 14:50
That's a good point. `->` is documented to have a higher precedence than `->*` indeed. I added it, but still personally prefer the extra parentheses for readability. – Ghislain Fourny Jun 21 '17 at 14:52
Actually, I would split it: `const auto fp = it->second; (this->*fp)();`, pointers-to-member-functions are so insanely complicated, that it is worth spelling it out. – Martin Bonner supports Monica Jun 21 '17 at 16:23

c++ calling function when using map of function pointer

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