C++ - one function makes others malfunction - c++

Question

Here is my task:
Write class Word which has:
pointer on array of characters
constructors and destructors
function to read word
function to check if character which is passed to it as argument occurs in word and return position of occurance
function to check which of two words has more occurances of number 10 and to return that number of occurances
Here is my solution. I compiled it without errors but it doesn't work as it shoud.
#include <iostream>
#include <string.h>
using namespace std;
class Word
{
private:
char *content;
int length;
public:
Word();
Word(char *);
~Word();
void print_content(void);
int check_character(char);
friend int check_number(Word,Word);
};
Word::Word()
{
}
Word::Word(char *n)
{
length=strlen(n);
content=new char [length];
for(int i=0;i<length;i++)
{
content[i]=n[i];
}
}
Word::~Word()
{
delete content;
}
void Word::print_content(void)
{
for(int i=0;i<length;i++)
{
cout<<""<<content[i];
}
}
int Word::check_character(char a)
{
int position=0;
for(int i=0;i<length;i++)
{
if(content[i]==a)
{
position=i+1;
}
}
if(position>0)
{
return position;
}
else return 0;
}
int check_number(Word n,Word m)
{
int counter_n=0;
int counter_m=0;
for(int i=1;i<n.length;i++)
{
if((n.content[i-1]=='1')&&(n.content[i]=='0'))
{
counter_n=counter_n+1;
}
}
for(int i=1;i<m.length;i++)
{
if((m.content[i-1]=='1')&&(m.content[i]=='0'))
{
counter_m=counter_m+1;
}
}
if(counter_n>counter_m)
{
return counter_n;
}
else if(counter_n<counter_m)
{
return counter_m;
}
else return 0;
}
int main()
{
char characters1[]="qwerty10",*p1,*p2;
char characters2[]="stackoverflow101010";
p1=characters1;
p2=characters2;
Word first(p1);
Word second(p2);
cout<<""<<first.check_character('q')<<endl;
cout<<""<<second.check_character('f')<<endl;
//cout<<""<<check_number(first,second)<<endl;
first.print_content();
second.print_content();
}
Function check_number(first,second) for some reason makes other functions to work incorrectly, if you call it by removing "//" you will see that first.print_content() and second.print_content() don't give us correct result. Or if function first.check_character('r') is first called, second.check_character('j') second called and then check_number(first,second), then two firsly called functions don't work.
What's reason for this strange behaviour?

Answer 1

Word objects are passed by copy to check_number, but you did not define the copy constructor. So default one is used by the compiler and this one copies pointer (char* content). Temporary objects passed to the function are pointing to data created first and second in the main function...upon deletion (temprary objects are deleted after the function is called), they delete the pointers, so first and second objects are pointing to deleted memory. You have undetermined behaviour here, this explains side effects you experienced.
Passing objects by reference to check_number is an easy way to fix your problem. Here is a working code (including many fixes because you did not access arrays correctly):
#include <iostream>
using namespace std;
#include <iostream>
#include <string.h>
using namespace std;
class Word
{
private:
char *content;
int length;
public:
Word();
Word(char *);
~Word();
void print_content(void);
int check_character(char);
friend int check_number(const Word&,const Word&); // changed here
};
Word::Word()
{
}
Word::Word(char *n)
{
length=strlen(n);
content=new char [length];
for(int i=0;i<length;i++)
{
content[i]=n[i]; // changed here
}
}
Word::~Word()
{
delete [] content; // changed here
}
void Word::print_content(void)
{
for(int i=0;i<length;i++)
{
cout<<""<<content[i]; // changed here
}
}
int Word::check_character(char a)
{
int position=0;
for(int i=0;i<length;i++)
{
if(content[i]==a) // changed here
{
position=i+1;
}
}
if(position>0)
{
return position;
}
else return 0;
}
int check_number( const Word& n, const Word& m)// changed here
{
int counter_n=0;
int counter_m=0;
for(int i=1;i<n.length;i++)
{
if((n.content[i-1]=='1')&&(n.content[i]=='0')) // changed here
{
counter_n=counter_n+1;
}
}
for(int i=1;i<m.length;i++)
{
if((m.content[i-1]=='1')&&(m.content[i]=='0')) // changed here
{
counter_m=counter_m+1;
}
}
if(counter_n>counter_m)
{
return counter_n;
}
else if(counter_n<counter_m)
{
return counter_m;
}
else return 0;
}
int main()
{
char characters1[]="qwerty10",*p1,*p2;
char characters2[]="stackoverflow101010";
p1=characters1;
p2=characters2;
Word first(p1);
Word second(p2);
cout<<""<<first.check_character('q')<<endl;
cout<<""<<second.check_character('f')<<endl;
cout<<""<<check_number(first,second)<<endl;
first.print_content();
second.print_content();
}
This outputs:
1
10
3
qwerty10stackoverflow101010
Declaring a copy constructor is another way to fix the problem:
Word( const Wodr& word ) :
length( word.length ),
content( new char[word.length] )
{
memcpy( content, word.content, word.length );
}
That would be less efficient than passing objects by const reference, but would make your code safer (it's good to always declare copy constructor to prevent bug you experienced here).
Finally, if you are lazy, you can also declare the copy constructor as private, to prevent compiler to copy objects, just declare it, don't impelment it:
class Word
{
....
private:
Word( const Word& word ); // this makes argument passed by copy impossible.
};
Then, compiler will not let you call check_number.