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#include <iostream>
using namespace std;
struct Card
{};
 
struct CardDeck
{
	CardDeck(const Card& card);
    friend CardDeck operator+(const CardDeck&, const CardDeck&);
};
 
CardDeck operator+(const CardDeck&, const CardDeck&);
 
int main() 
{
	Card c1;
	CardDeck cd(c1);
	Card myCard;
	CardDeck cd2 = myCard + myCard;
	return 0;
}

/home/wZlnDx/ccQ0OJTh.o: In function `main':
prog.cpp:(.text.startup+0x22): undefined reference to `CardDeck::CardDeck(Card const&)'
prog.cpp:(.text.startup+0x2b): undefined reference to `CardDeck::CardDeck(Card const&)'
prog.cpp:(.text.startup+0x37): undefined reference to `CardDeck::CardDeck(Card const&)'
prog.cpp:(.text.startup+0x45): undefined reference to `operator+(CardDeck const&, CardDeck const&)'
collect2: error: ld returned 1 exit status

Standard output is empty