The C++ Copy Constructor

The C++ copy constructor is very important when working with objects. It allows for objects to be passed by value to function arguments and then returned from functions. If a class dynamically allocates memory then it must have a custom copy constructor written. (Also, the assignment operator should be overloaded.) All of the data structures on this site implement copy constructors:

#include <iostream>
using namespace std;

class CopyCounter
{
private:
	int copyCount;
public:
	CopyCounter() : copyCount(0) {}
	~CopyCounter() { cout << "destroy copy count " << copyCount << endl; }
	CopyCounter(const CopyCounter &obj) : copyCount(obj.copyCount + 1) // custom copy constructor
	{
		cout << "copy operation under way; copy number " << copyCount << endl;
	}
};

CopyCounter MakeCopies(CopyCounter byValue) // invoke copy constructor
{
	return byValue; // invoke copy constructor & destroy this copy
}

int main()
{
	CopyCounter c0;
	CopyCounter c1 = MakeCopies(c0); // this makes a few copies and destroys one copy
	CopyCounter c2 = MakeCopies(c1); // this makes a few copies and destroys one copy
	CopyCounter c3 = c2; // this just makes one copy (copy 5)
	return 0;
}

Here, the DynamicMem class is using dynamic memory and the copy constructor generated by the compiler does not know how to make a copy of it because it does not know the size of the data that str points to. Run this and there will be a violation.

#include <iostream>
using namespace std;

class DynamicMem
{
private:
	char *str;
public:
	DynamicMem()
	{
		cout << "creating *str" << endl;
		str = new char[4]; // make str == "abc"
		str[0] = 'a';
		str[1] = 'b';
		str[2] = 'c';
		str[3] = 0;
	}
	~DynamicMem()
	{
		cout << "deleting *str" << endl;
		delete[]str;
	}
	const char *get() const
	{
		cout << "getting *str" << endl;
		return str;
	}
};

DynamicMem MakeCopies(DynamicMem byValue) // invoke copy constructor
{
	return byValue; // invoke copy constructor & destroy this copy
}

int main()
{
	DynamicMem dm0;
	cout << dm0.get() << endl;
	DynamicMem dm1 = MakeCopies(dm0);
	cout << dm0.get() << endl;
	cout << dm1.get() << endl;
	return 0;
}

The key is to make a custom copy constructor and not rely on the one provided by the compiler. Run this code. Now it works without a violation.

#include <iostream>
using namespace std;

class DynamicMem
{
private:
	char *str;
public:
	DynamicMem()
	{
		cout << "creating *str" << endl;
		str = new char[4]; // make str == "abc"
		str[0] = 'a';
		str[1] = 'b';
		str[2] = 'c';
		str[3] = 0;
	}
	DynamicMem(const DynamicMem &obj) // CUSTOM COPY CONSTRUCTOR
	{
		cout << "creating *str in custom copy constructor" << endl;
		str = new char[4]; // make str == obj.str
		*(int*)str = *(int*)obj.str; // this is a little trick to fit the whole string in a register
	~DynamicMem()
	{
		cout << "deleting *str" << endl;
		delete[]str;
	}
	const char *get() const
	{
		cout << "getting *str" << endl;
		return str;
	}
};

DynamicMem MakeCopies(DynamicMem byValue) // invoke copy constructor
{
	return byValue; // invoke copy constructor & destroy this copy
}

int main()
{
	DynamicMem dm0;
	cout << dm0.get() << endl;
	DynamicMem dm1 = MakeCopies(dm0);
	cout << dm0.get() << endl;
	cout << dm1.get() << endl;
	return 0;
}

Alternatively, the string class could have been used here and then there would be no need to write a custom copy constructor. This is because the string class has its own custom copy constructor as it uses dynamic memory.

#include <string>
#include <iostream>
using namespace std;

class DynamicMem
{
private:
	string str;
public:
	DynamicMem() : str("abc")
	{
		cout << "creating str" << endl;
	}
	const string &get() const // notice that this method returns a reference to the string object "str"
	{
		cout << "getting str" << endl;
		return str;
	}
};

DynamicMem MakeCopies(DynamicMem byValue) // invoke copy constructor
{
	return byValue; // invoke copy constructor & destroy this copy
}

int main()
{
	DynamicMem dm0;
	cout << dm0.get() << endl;
	DynamicMem dm1 = MakeCopies(dm0);
	cout << dm0.get() << endl;
	cout << dm1.get() << endl;
	return 0;
}

"Privatize" the Copy Constructor

Make a copy constructor private to prevent the object from being copied.

#include <iostream>
using namespace std;

class CopyCounter
{
private:
	int copyCount;
	CopyCounter(const CopyCounter &obj) {} // private custom copy constructor
public:
	CopyCounter() : copyCount(0) {}
};

CopyCounter MakeCopies(CopyCounter byValue) // invoke copy constructor
{
	return byValue; // invoke copy constructor
}

int main()
{
	CopyCounter copy;
	MakeCopies(copy); // can't do this
	CopyCounter copy2 = copy; // can't do this
	return 0;
}

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