copy constrctor

[venkatapr]

i defined a class sample like this..


class sample
{ int i;
public:
explicit sample(int a)
{ i=a;
cout<<"i amin constructor"<<endl;
}

sample(sample &s)
{ i=s.i;
cout<<"i am in copy constructor"<<endl;
}
void display()
{ cout<<" i am in display";
}

};



and if i write this statement in the main
sample r=sample(8);

it is throwing following error...

explicitk.cpp:31:21: error: no matching function for call to ‘sample::sample(sample)’
explicitk.cpp:31:21: note: candidate is:
explicitk.cpp:14:1: note: sample::sample(sample&)
explicitk.cpp:14:1: note: no known conversion for argument 1 from ‘sample’ to ‘sample&’



please help me with this ...i am new to c++


thanks
Venkata prasad manne

[MrHutch]

A copy constructor is expecting a parameter of the same class type.

You're passing an integer.

Here's a simple example:

#include <iostream>

struct MyStruct
{
  int a;
  MyStruct():a(10){}
  MyStruct( const MyStruct &other )
  {
    this->a = other.a;
  }
};

int main( int argc, char* argv[] )
{
  MyStruct firstStruct; // Runs normal constructor, a is 10
  firstStruct.a = 4;    // Let's change the value
  MyStruct secondStruct( firstStruct ); // Copies the first struct

  std::cout << "Value of a in secondStruct is " << secondStruct.a << std::endl;

  return 0;
}

[ajh32]

Yes, he's calling the custom constructor which will work. Also, using his code if he were to call the copy constructor, he shouldn't get the error he is apparently getting. If I just copy his code it all compiles and works and if I just add a further line to the main function:

sample r2 = sample(r);

All still compiles and works fine!

[vlad from moscow]

The problem is that you defined the copy constructor as haveing as the parameter non-const reference to class object.
In this statement

sample r=sample(8);

at first a temporary object of type sample is created using constructor explicit sample( int ):

sample( 8 )

A temporary object can be binded with const reference. So apart from the copy constructor sample( sample & ) you need copy constructor sample( const sample &). Your code could be compiled if you would write

sample t( 8 );
sample r=t;

In this case your copy constructor would be called because t is not a temporary object.

It is enough to have only one copy constructor defined as
sample( const sample & )
It will be called for temporary and non-temporary objects.

[MrHutch]

@ajh32

Whoops, my bad. I missed the constructor with the integer parameter there. :-S

[venkatapr]

THanks evryone for replying .....and thanks vaidfrommosow for ur exact answer..