operator overloading not working

I am new to C++ plz help

The below code does not work.. (printing garbage value )can any 1 explain y....

Code:

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

struct bignum
{
	char digits[1000];
	int lastdigit;
	
	bignum operator = (char *str)
	{	
		int j=0;
		bignum temp;
		temp.lastdigit=strlen(str)-1;
		
//		printf("len=%d\n",temp.lastdigit);	/* working fine over here */
		
		for(int i=temp.lastdigit;i>=0;i--)
			temp.digits[i]=str[j++];
		
		return temp;
	}
}; 
	
	
int main()
{
	bignum num;
	char s[10]="12345";
	
	num=s;
	printf("%d\n",num.lastdigit);  /* why printing garbage value ?? */ 
	
return 0;
}

And also when operator overload is declared outside the struct i get a compile error saying

Code:

addition.cpp:12: error: ‘bignum operator=(char*)’ must be a nonstatic member function
addition.cpp: In function ‘int main()’:
addition.cpp:32: error: no match for ‘operator=’ in ‘num = s’
addition.cpp:7: note: candidates are: bignum& bignum::operator=(const bignum&)

Following is the code that gives compile error

Code:

#include <iostream>
#include <algorithm>
#include <string.h>
using namespace std;

struct bignum
{
	char digits[1000];
	int lastdigit;
}; 

bignum operator = (char *str)
{	
	int j=0;
	bignum temp;
	temp.lastdigit=strlen(str)-1;
		
//	printf("len=%d\n",temp.lastdigit);	/* working fine over here */
		
	for(int i=temp.lastdigit;i>=0;i--)
		temp.digits[i]=str[j++];
		
	return temp;
}

	
int main()
{
	bignum num;
	char s[10]="12345";
	
	num=s;
	printf("%d\n",num.lastdigit);  /* why printing garbage value ?? */ 
	
return 0;
}

Thank u

1) You are assigning to a temporary object and not to the real object! You can't expect it to work when you are doing the actual assignment to a temporary object and returning it. The state of the class (or struct) itself doesn't change.
2) Free members are different. Firstly they must be friend and secondly the left-hand argument (the first argument it takes) must be an instance of the class that it manipulates by reference.

MyClass& operator = (MyClass& lhs, const T& rhs)

Also note that the argument that operator = takes should be const, because it doesn't change it.

but i am also returning the temporary object...so shouldn't it go into num (in main) ???

Yes, you are returning it but not using it. You would be using it if you assigned it to something else, a la:

Code:

Bignum a, b;
a = b = "1234";

Now a would be the temporary returned from b.

Most often result of operator= is just ignored.

No.
Operator = changes the state of the left-hand side, so in this case:
num=s
Num should be changed.
Unfortunately it isn't. Instead the expression
(num=s)
returns a temporary object which is discarded (and it's also wrong).

Also, operator = usually returns a reference itself:
return *this;
This allows stuff to be chained, such as:
a = b = c = d;
etc

You can have the = operator do whatever you like. You can have it multiply instead of assign, or even launch a rocket, C++ will let you write whatever you want as the definition.

If you want it to do an assignment, the reasonable expectation for its use, then you need to change the original value, not the temporary variable.

Fixes:
str should be const char *
return type should be bignum&
remove the temp variable and all occurrences of temp.
return *this
You can also lose the .h

Next step:
Implement the default constructor and a constructor that also takes a const char * as an argument.