Reducing code duplication between operator= and the copy constructor
I have a class that requires a non-default copy constructor and assignment operator (it contains lists of pointers). Is there any general way to reduce the code duplication between the copy constructor and the assignment operator?
There's no "general way" for writing custom copy constructors and assignment operators that works in all cases. But there's an idiom called "copy-&-swap":
class myclass
{
...
public:
myclass(myclass const&);
void swap(myclass & with);
myclass& operator=(myclass copy) {
this->swap(copy);
return *this;
}
...
};
It's useful in many (but not all) situations. Sometimes you can do better. A vector or a string could have a better assignment which reuses allocated storage if it was large enough.
Factor out the common code to a private member function. A simple (rather contrived) example:
#include <iostream>
class Test
{
public:
Test(const char* n)
{
name = new char[20];
strcpy(name, n);
}
~Test()
{
delete[] name;
}
// Copy constructor
Test(const Test& t)
{
std::cout << "In copy constructor.n";
MakeDeepCopy(t);
}
// Assignment operator
const Test& operator=(const Test& t)
{
std::cout << "In assignment operator.n";
MakeDeepCopy(t);
}
const char* get_name() const { return name; }
private:
// Common function where the actual copying happens.
void MakeDeepCopy(const Test& t)
{
strcpy(name, t.name);
}
private:
char* name;
};
int
main()
{
Test t("vijay");
Test t2(t); // Calls copy constructor.
Test t3("");
t3 = t2; // Calls the assignment operator.
std::cout << t.get_name() << ", " << t2.get_name() << ", " << t3.get_name() << 'n';
return 0;
}
My &My::operator = (My temp) // thanks, sellibitze
{
swap (*this, temp);
return *this;
}
并实现一个专门的std::swap<> (My &, My &) 。
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