How do I get a value from a class table in C++

Question

Basically, I have a class in C++

class Commands { public: void help(); void userid(); void getCmd(std::string cmd) { } };
void Commands::help() {
    WriteLine("\n \
        --------------------------------\n \
        [ userid, id ]: Get's the UserID of your player\n \
        [ placeid, game ]: Get's the game ID\n \
        --------------------------------\n \
        \
    ");
}
void Commands::userid() {
    WriteLine("Your UserId is: 12345678");
}

I have a while(true) loop that runs a function which checks what the user responds with [ like a command based console ] I was wondering how I could do this:

Commands cmds;
cmds["help"]();
cmds["examplecmdhere"]();

When I try to do it I get an error Severity Code Description Project File Line Suppression State Error (active) E0349 no operator "[]" matches these operands Command

[`std::map`](https://en.cppreference.com/w/cpp/container/map) or [`std::unordered_map`](https://en.cppreference.com/w/cpp/container/unordered_map) and [`std::function`](https://en.cppreference.com/w/cpp/utility/functional/function). Eg `std::map> cmds = { {"help",[this](){ help(); } } };` — user4581301, Jul 16 '22 at 04:39
@user4581301 Are you able to answer the edited question, please? — RealLinen, Jul 16 '22 at 04:55

user4581301 · Accepted Answer · 2022-07-16T05:16:21.773

Use std::map or std::unordered_map to map std::strings to std::function that represent the functions to be called, then overload operator[] to provide easy access to the map without exposing the map.

#include <string>
#include <map>
#include <functional>
#include <iostream>
class Commands { 
    // added: a map of strings to the function to call. private to keep 
    // unwanted hands off
    std::map<std::string, std::function<void()>> cmds = { 
        {
            "help", // Name of command
            [this](){ help(); } //Lambda expression that captures instance pointer
                                // and uses it to invoke help function 
        } 
    };
public: 
    void help(); 
    void userid(); 
    void getCmd(std::string cmd) { } 
    // added: overload the [] operator to give access to the map defined earlier
    auto & operator[](const std::string & cmd)
    {
        return cmds[cmd];
    }
private:

};
void Commands::help() {
    // I have no WriteLine function, so I'm subbing in `std::cout for 
    // demonstration purposes
    std::cout << "\n \
        --------------------------------\n \
        [ userid, id ]: Get's the UserID of your player\n \
        [ placeid, game ]: Get's the game ID\n \
        --------------------------------\n \
        \
    ";
}

int main()
{
    Commands commands;
    commands["help"](); //use operator[] to call help function
}

Result:


         --------------------------------
         [ userid, id ]: Get's the UserID of your player
         [ placeid, game ]: Get's the game ID
         --------------------------------

See it in action: https://godbolt.org/z/Kh39jqq3T

How do I get a value from a class table in C++

1 Answers1