When do we have to use copy constructors?

Question

I know that C++ compiler creates a copy constructor for a class. In which case do we have to write a user-defined copy constructor? Can you give some examples?

Answer 1

The copy constructor generated by the compiler does member-wise copying. Sometimes that is not sufficient. For example:

class Class { public:     Class( const char* str );     ~Class(); private:     char* stored; };  Class::Class( const char* str ) {     stored = new char[srtlen( str ) + 1 ];     strcpy( stored, str ); }  Class::~Class() {     delete[] stored; } 

in this case member-wise copying of stored member will not duplicate the buffer (only the pointer will be copied), so the first to be destroyed copy sharing the buffer will call delete[] successfully and the second will run into undefined behavior. You need deep copying copy constructor (and assignment operator as well).

Class::Class( const Class& another ) {     stored = new char[strlen(another.stored) + 1];     strcpy( stored, another.stored ); }  void Class::operator = ( const Class& another ) {     char* temp = new char[strlen(another.stored) + 1];     strcpy( temp, another.stored);     delete[] stored;     stored = temp; }