错误：&#39;std::__cxx11::list<User> ::iterator&#39; {aka &#39;struct std::_List_iterator<User> &#39;} 没有名为 XXX 的成员

Question

I am learning C++ but I get an error with my first class. I need help to understand what it is wrong with this code.

I get the following error message:

main.cpp:36:13: error: 'std::__cxx11::list<User>::iterator' {aka 'struct std::_List_iterator<User>'} has no member named
 'printInfo'
         usr.printInfo();
             ^~~~~~~~~

The main function prompts users to enter their name, creates list and list iterator, displays Info of the two users

The main() function:

#include <iostream>
#include <list>
#include "User.cpp"
using namespace std;
int main() {
    string str0 ="";
    usrs players; 
    players = create_2user();
    list<User> playerList = { players.usr0, players.usr1 };
    for( list<User>::iterator usr = playerList.begin(); usr != playerList.end(); usr++) {
        usr.printInfo();
    }       
   return 0;
}

The class User :

class User {
    string name;
    public:
        User();      
        void set_name (string in_name);     
        string get_name();
        void printInfo();
};
User::User()  { name="Unkonw"; }
void User::set_name(string in_name) { name  = in_name; }
string User::get_name() { return name; }
void   User::printInfo() { cout<<name; }

struct usrs { User usr0, usr1; };

usrs create_2user() {
    User array_usr[2];
    string str0;

    for(int i=0;i<2;i++) {
        cout<<"Enter player "<<i<<"'s name: ";
        cin>>str0;
        array_usr[i].set_name(str0);
    }
    usrs result = { array_usr[0], array_usr[1] };
    return result;
}

Answer 1

You try to invoke the member function printInfo on an iterator to a player instance, not a user instance. You can fix this by

for( list<User>::iterator usr = playerList.begin(); usr != playerList.end(); usr++) {
    usr->printInfo();
}

or with a range base for loop, which does the dereferencing for you:

for (User& usr : playerList)
    usr.printInfo(); // Here, your original syntax works

If you want to overengineer it, you could also begin learning the <algorithm> header and

#include <algorithm>

std::for_each(playerList.cbegin(), playerList.cend(),
    std::mem_fn(&User::printInfo));

which in C++20 won't be as absurd for a beginner as the above usage:

std::ranges::for_each(playerList, &User::printInfo);

Note that as printInfo() doesn't alter any state of the object, you should mark this member function const , and use a const_iterator , within the range base for loop, for (const User& usr : playerList) and for the algorithm invocation cbegin()/cend() .

Answer 2

This: list<User>::iterator usr declares usr to be an iterator. In C++, an iterator is something like a generalised pointer — an object which points to another object ⁽¹⁾ . So you have to treat it like a pointer: if you want to access the object being pointed to, you have to derefernce the iterator using either * (to get to the object), or -> (to get to the object's members). So change the statement in the for loop body to:

usr->printInfo();

---

⁽¹⁾ The difference is that pointers point to an address, while iterators point to a position in a container.