Heap corruption detected

[icapri77]

I have

"HEAP CORRUPTION DETECTED after normal block at

CRT DETECTED that the application wrote to memory after end of heap buffer"

error Message

After I debugged I found that this error message occured at ~String() method(delete problem)
plz help me to corect this error.

this is the full code ################################   strng3.h

#ifndef _STRNG2_H_
#define _STRNG2_H_

class String
{
private:
	char *str;
	int len;
public:
	String(const char *s);
	String();
	String(const String &st);
	~String();
	int length() const {return len;}

	String & operator=(const String &st);
	String & operator=(const char *s);

	String & operator+(const String &st);
	String & operator+(const char *s);

	friend ostream & operator<<(ostream &os,const String &st);
	friend istream & operator>>(istream &is,String &st);
	friend String & operator+(const char *s,String &st);
};

#endif


################################ strng3.cpp

#include <iostream>
#include <cstring>
using namespace std;
#include "strng3.h"

String::String()
{
	len=0;
	str=new char[1];
	str[0]='\0';

}

String::String(const char *s)
{
	len=strlen(s);
	str=new char[len+1];
	strcpy(str,s);
}

String::String(const String &st)
{
	len=st.len;
	str=new char[len+1];
	strcpy(str,st.str);
}

String::~String()
{
	delete [] str;               // <-------------------------------------- here
}

String & String::operator =(const char *s)
{
	delete [] str;
	len=strlen(s);
	str=new char[len+1];
	strcpy(str,s);
	return *this;
}

String & String::operator =(const String &st)
{
	if(this==&st)
		return *this;
	delete [] str;
	len=st.len;
	str=new char[len+1];
	strcpy(str,st.str);
	return *this;
}

String & String::operator +(const String &st)
{
	for(int i=0;i<=st.len;i++)
		*(str+len+i)=*(st.str+i);
	len=len+st.len;
	return *this;	
}

String & String::operator +(const char *s)
{
	int slen=strlen(s);
	char * temp=new char[slen+1];
	strcpy(temp,s);
	for(int i=0;i<slen;i++)
		*(str+len+i)=*(temp+i);
	delete [] temp;
	return *this;
}

String & operator+(const char *s,String &st)
{
	String temp(s);
	st=temp+st;                    //<---------------------- here
	return st;
}


ostream & operator<<(ostream &os,const String &st)
{
	os<<st.str;
	return os;
}

istream & operator>>(istream &is,String &st)
{
	char temp[80];
	is.get(temp,80);
	if(is)
		st=temp;
	while(is && is.get()!='\n')
		continue;
	return is;
	
	/*
	char temp[20];
	is>>temp;
	char * ps=new char[strlen(temp)+1];
	strcpy(ps,temp);
	strcpy(st.str,ps);
	st.len=strlen(st.str);
	delete [] ps;
	return is;
	*/
}

#############################################

#include <iostream>
using namespace std;
#include "strng3.h"

int main()
{
	String s1("and I'm learnging c++");
	String s2="your name:";
	String s3;
	cout<<s2;
	cin>>s3;
	s2="my name is "+s3;            //<--------------- here
	cout<<s2<<".\n";
	s2=s2+s1;
	cout<<s2<<"\n";

	return 0;
}

[guestgulkan]

Your operator+ overloads are seriously flawed.

[Disch]

To elaborate a bit more:

* + operators should be const, and should not change 'this'. If you have code that does c = a + b;, you don't expect 'a' to change, however with your operators 'a' would change. + operators should also return an object by value, not a reference.

* Your + overload on line 97 doesn't increase the size of the buffer to accomidate the new, longer string. Therefore you get buffer overflows / heap corruption.

* Your + overload on line 105 changes the size of the buffer, but only to make it long enough to hold the new string -- it does not make it large enough to hold both the new string and the old string (ie: it'd be find for assignment, but for appending, it's no good). So more heap corruption there.

[icapri77]

Thank for all your help.

I found the solution.

Now it works with no heap corruption!

String & String::operator +(const String &st)
{
	char *temp=new char[len+st.len+1];
	strcpy(temp,str);
	for(int i = 0; i <= st.len; i++)
	{
		*(temp + len + i) = *(st.str + i);
	}
	len = len + st.len;
	delete [] str;
	str=new char[len+1];
	strcpy(str,temp);
	delete [] temp;
	return *this; 

}

String operator+(const char *s,String &st)
{
	String temp(s);
	return temp+st;
}