How to pass a vector of a Child class in to a function expecting a vector of Parent class?

Question

I can pass in a Child to a member function expecting a Parent, however when using vectors I get a compile error saying there's no matching declaration. See the CorrelationEngineManager.cpp call to getUniqueLabels() at the bottom

ServerEvent.h

#ifndef SERVEREVENT_H
#define SERVEREVENT_H

#define SERVEREVENT_COLS 3

#include "Event.h"
#include <vector>


class ServerEvent: public Event {
private:

public: 
    ServerEvent(std::vector<std::string> tokens);
    void print();
};

#endif

Event.h

#ifndef EVENT_H
#define EVENT_H

#include <string>

#define EVENT_STOP 0
#define EVENT_START 1

class Event {
private:

protected:
    double time;
    std::string label;
    int type; // EVENT_START OR EVENT_STOP

public:

};

#endif

CorrelationEngineManager.h

class CorrelationEngineManager {
private:
    std::vector<ServerEvent> s_events;
    std::vector<UPSEvent> u_events;
    std::vector<TimeRecord> s_timeRecords;
    std::vector<TimeRecord> u_timeRecords;
    // typeOfEvent gets type of event, 0 for error, look at #defines for codes
    int typeOfEvent(std::vector<std::string>);
    int createTimeRecords();
    std::vector<std::string> getUniqueLabels(std::vector<Event> events);


public:
    CorrelationEngineManager();
    //~CorrelationEngineManager();
    int addEvent(std::vector<std::string> tokens); //add event given tokens
    void print_events();
};

CorrelationEngineManager.cpp

int CorrelationEngineManager::createTimeRecords() {
    std::vector<std::string> u_sLabels; // unique server labels
    std::vector<std::string> u_uLabels; // unique UPS labels
    u_sLabels = getUniqueLabels(s_events);
//  u_uLabels = getUniqueLabels(u_events);
    return 1;
}
// returns a vector of unique labels, input a vector of events
std::vector<std::string> CorrelationEngineManager::getUniqueLabels(std::vector<Event> events) {

    std::vector<std::string> temp;
    return temp;
}

compile error

 CorrelationEngineManager.cpp: In member function ‘int CorrelationEngineManager::createTimeRecords()’:
 CorrelationEngineManager.cpp:60: error: no matching function for call
 to ‘CorrelationEngineManager::getUniqueLabels(std::vector<ServerEvent,
 std::allocator<ServerEvent> >&)’ CorrelationEngineManager.h:23: note:
 candidates are: std::vector<std::basic_string<char,
 std::char_traits<char>, std::allocator<char> >,
 std::allocator<std::basic_string<char, std::char_traits<char>,
 std::allocator<char> > > >
 CorrelationEngineManager::getUniqueLabels(std::vector<Event,
 std::allocator<Event> >) make: *** [CorrelationEngineManager.o] Error 1

Answer 1

This is not possible in C++, this requires a feature called covariance.

Even if type A is a subclass of type B, type X<A> is completely unrelated to type X<B>

Thus you cannot pass std::vector<UPSEvent> to a function expecting std::vector<Event>, since they are unrelated types. Even pass by reference/pointer will not work.

There are two ways to get around this.

One would be to make both vectors hold pointers to Event, then they would have identical types.

The other, would be to make the function a template function, as Daniel suggests.

You would need to fix the signature as well, as billz points out.

Answer 2

The function could be changed to a template function:

template< typename T >
std::vector<std::string> getUniqueLabels(std::vector<T> events);