I want to create a vector with diferent object of type A but ...
This code throws this exception:
free(): invalid pointer: 0x0000000000401757 ***
#include <vector>
class A
{
char * elem;
public:
A()
{
elem = new char[10];
}
void setA(char* name)
{
elem = name;
}
~A()
{
delete[] elem;
}
};
int main(int argc, char **argv)
{
std::vector<A> v_a;
for (int i = 0; i < 10; ++i)
{
A m_a;
m_a.setA("jaja");
v_a.push_back(m_a);
}
}
Why this is happening?? How can implement a correct solution?
When calling setA(...) you are not copying the data of the string, instead you are copying the address of a local string. (Check what cplusplus.com says about pointers and string literals)
After the block with the setA(...) call ends, this string goes out of scope so it's address is invalid afterwards
So you are trying to free memory from the heap in your destructor which was allocated on the stack and is long gone at this time...
Updated suggestion 1: added on Jan 27th, 2016
additionally take into account the need of a copy ctor when pushing instances of A to a std::vector. Obeying the Rule of three I added a copy assignment operator as well.
See also Rule of five in case of c++11
properly take care of possibly 'wrong' sized strings
#include <cstring>
...
/// copy ctor
A(const A& other) :
elem(new char[10])
{
// prevent duplicate code
setA(other.elem);
}
/// copy assignment operator
A& operator=(const A& other)
{
// prevent duplicate code
setA(other.elem);
return *this;
}
/// set A's name (which is truncated to 9 characters if necessary)
void setA(const char* name)
{
// copy first 9 chars from name
// if there's fewer chars: elem is padded with zeros
std::strncpy(elem, name, 9);
// add null character / zero (string delimiter) manually
// this is necessary for cases where name has 10 or more chars
elem[9] = 0;
}
(Obsolete) suggestion 1: continue working with c style strings
explicitly copy the passed string's data (you need to do this in order to make the method work)
add a length indicator to the parameter list of setA(...) (this is optional, you could also figure out the length of the string inside of the method...)
#include <cstring>
...
void setA(char* name, size_t length)
{
std::memcpy(elem, name, length);
}
Suggestion 2: switch to c++ style strings
use std::string (I'd prefer this version as it spares you the memory handling and reads very intuitively)
#include <string>
class A
{
std::string elem;
public:
A(){}
void setA(std::string name)
{
elem = name;
}
~A(){}
};
You modify the value of elem, which is a pointer to a memory area allocated on the free store. Now, elem points to a string literal, which is not applicable to delete or delete[].
deleteing objects not on the free store is undefined behavior. Your destructor attempts it nevertheless.
Related
Multiple students can associate with a single Department and single student can
associate with multiple Departments, but there is no ownership between the objects
and both have their own lifecycle. Both can create and delete independently.
WAP in C++ to model the relationships.
I have implemented this code as follows
#include<iostream>
#include<cstring>
using namespace std;
class Student
{
char* name_p;
public:
Student(char *sName)
{
cout<<"Student constructor called\n";
name_p=new char(sizeof(strlen(sName)));
name_p=sName;
}
~Student()
{
cout<<"Student destructor called\n";
delete name_p;
};
char* sName()
{
return name_p;
}
};
class Department
{
char* name_p;
public:
Department(char *dName)
{
cout<<"Department destructor called\n";
name_p=new char(sizeof(strlen(dName)));
name_p=dName;
}
~Department()
{
cout<<"Department destructor called\n";
delete name_p;
}
char* dName()
{
return name_p;
}
};
class Course
{
Student* std_p;
Department* dept_p;
char* courseName_p;
static unsigned int index;
static Course *courseList_p[4];
public:
Course(char* crseName,Student* student,Department* dept)
{
cout<<"Course constructor called\n";
std_p=student;
dept_p=dept;
if(index<4)
{
courseName_p=new char(sizeof(strlen(crseName)));
courseName_p=crseName;
courseList_p[index]=this;
++index;
}
else
{
cout<<"Cannot accomodate any more Course\n";
}
};
~Course()
{
cout<<"Course destructor called\n";
delete courseName_p;
};
static char* findStudent(char *crseName, char* deptName)
{
for(int i=0; i<index; i++)
{
if ( (courseList_p[i]->getCourseName() == crseName) &&
(courseList_p[i]->getDeptName() == deptName) )
{
return(courseList_p[i]->getStdName());
}
}
}
char* getStdName()
{
return std_p->sName();
};
char* getDeptName()
{
return dept_p->dName();
};
char* getCourseName()
{
return courseName_p;
};
};
unsigned int Course::index =0;
Course* Course::courseList_p[4]={0,0,0,0};
int main()
{
int i;
cout<<"\nExample of Association class\n";
cout<<"-----------------------------------\n\n";
cout<<"We have got 4 students\n";
Student *studentNames[4] = {new Student("Meera"), new Student("Moina"), new Student("Teena"), new Student("Mridula")} ;
cout<<"\n";
cout<<"We have got 2 Departments\n";
Department *departNames[2] = {new Department("Mathematics"), new Department("ComputerSceince")} ;
cout<<"\n";
cout<<"Here class Course Associates Student and Department, with a Course name\n";
Course course1("DataStructure",studentNames[0], departNames[1]);
Course course2("Maths",studentNames[3], departNames[0]);
Course course3("Geometry",studentNames[2], departNames[0]);
Course course4("CA",studentNames[1], departNames[1]);
cout<<"\n";
cout<<"Finding a Student using Course and Department\n";
cout<<"Student who has taken Maths Course in Mathematics Department is:"<<Course::findStudent("Maths", "Mathematics")<<endl;
cout<<"\n";
cout<<"Deletion of objects\n\n";
for(i=0;i<4;i++)
{
delete studentNames[i];
}
cout<<"\n";
for(i=0;i<2;i++)
{
delete departNames[i];
}
cout<<"\n";
return 0;
}
The code is showing warnings in main function i.e. ISO C++ forbids converting a string constant to char* and also the main() function is not returning 0 but a garbage value. Please help me in rectify these errors and warnings.
Also I don't want to use this pointer in class course, can I implement the code without using this pointer.
There are a couple of problems with your code. Mostly they are related to the use of char* instead of string. I'm amazed to see that some teacher still are found of this practice.
c-string allocation problems
The first problem causes memory corruption. This could explain that your main() returns garbage instead of 0. For example:
name_p=new char(sizeof(strlen(dName))); // OUCH!!!
In the statement you allocate it wrong. If you want to allocate an array, you must use new[...]. Moreover, keep in mind that a c-string allways needs 1 char more for the trailing null. So it should be:
name_p=new char[strlen(dName)+1];
would already improve the situation.
Last but not the least, if you allocate an array you must delete an array with delete[]
c-string copying issue
Then, you can copy C++ string with an assignment operator just as in:
name_p=sName; // OUCH
However, for char*, this just overwrites the pointer that new returned and you are not sure that the c-string it points to will still be valid when it's used.
If you want to assign a c-string, you need to go for:
strcpy (name_p, sName); // C-style
copy (sName, sName+strlen(sName)+2, name_p); // Safer C++ alternative
c-string comparison issue
Suppose you have two pointers to c string, say pa and pb. Comparing the pointers looks at the memory address, not at the content. So yu could have two identical strings but the comparison could nevertheless fail. THis kind of comparison would work perfectly well for c++ string, but for cstring, you need to to
strcmp(pa, pb)==0 // instead of pa == pb
c-strings in classes and structures
Whenever you have some pointers that you allocate in a class, you need to think of the rule of 3. Meaning that you'd need to add a copy constructor and an assignment operator. Fortunately, at first sight it seems that you don't need them here. Of couse, if you'd use c++ string you wouldn't have to worry about such details.
c-string conversion issues?
ANother issue in your code is the use of string literals with constructors. For example:
new Student("Meera").... // c-string literals (constant)
Student(char*...) // constructor using a c-string pointer (non constant)
You can easily get rid of this complaint, by making the constructor's pointer argument as constant:
Student(const char *sName)...
The same applies to function arguments, such as in find.
Other problems
The static member findStudent() works well, but only if a result is found. If nothing is found, it's UB, since there is no return statement (put a return nullptr; at the end of this function?)
Moreover, the wya you use this function, directly printing its result is dangerous, because dereferencing an invalid pointer will lead to UB.
Here the corrected code. It compiles and seem to run. I didn't analyse for more issues.
I'm trying to create a constructor in which the strings are dynamically allocated. I've looked up dynamically allocated memory several times and watched a video about it, but I'm still not 100% sure if I'm understanding the concept. I'm hoping an example specific to what I'm coding will help me out a bit.
These are the private variables I have in my h file:
string* tableID;
int numSeats;
string* serverName;
With that in mind, could someone tell me how I could dynamically allocate memory for the strings in this constructor?
Table::Table(const string& tableID, int numSeats, const string& serverName) {
}
Finally, I would greatly appreciate it if someone could tell me the purpose of dynamically allocated memory. I've see explanations on what dynamically allocate memory is, but I'm not understanding the use of it. Why use dynamically allocated memory? What are the benefits? What are the drawbacks? Thank you!
EDIT: I'm including the rest of the h file. Note that this wasn't created by me, so I can't make changes to it. I can only adhere to it in the cpp file.
#include <string>
#include "party.h"
using std::string;
class Table {
public:
Table();
Table(const string& tableID, int numSeats, const string& serverName);
~Table();
const string* getTableID() const { return tableID; }
int getNumSeats() const { return numSeats; }
const string* getServerName() const { return serverName; }
void decrementTimer() { timer--; }
int getTimer() const { return timer; }
void setTimer(int duration) { timer = duration; }
const Party* getParty() { return party; }
void seatParty(const Party* newParty);
void clearTable() { party = nullptr; timer = 0; }
private:
string* tableID;
int numSeats;
string* serverName;
int timer;
const Party* party;
};
The easiest way to get what you want is to take advantage of the Member Initializer List as this also solves the problem of having the parameters shadow the member variables of the same name.
Table::Table(const string& tableID,
int numSeats,
const string& serverName):
tableID(new string(tableID)),
numSeats(numSeats),
serverName(new string(serverName))
{
}
Allocation is performed with the new operator. Later you will have to release the dynamically allocated memory with the delete operator. Here is documentation on new and the same for delete.
But the use a pointer requirement is bizarre as storing pointers to string makes everything else you with the class do orders of magnitude more difficult. This may be the point of the assignment, but there are better and less-confusing ways to teach this lesson.
The allocated strings must be released. The C++ idiom of Resource Allocation Is Initialization (What is meant by Resource Acquisition is Initialization (RAII)?) suggests you have a destructor to automate clean-up to ensure that it is done. If you need a destructor, you almost always need the other two members of The Big Three (What is The Rule of Three?) and possibly need to take The Rule of Five into account as well.
Whereas because string observes the Rule of Five for you, you should be able to take advantage of the Rule of Zero and implement no special functions.
M.M raises an excellent point in the comments. The above example is too naive. It is probably all you need for the assignment, but it's not good enough for real code. Sooner or later it will fail. Example of how it fails.
First we replace string with something that can expose the error:
class throwsecond
{
static int count;
public:
throwsecond(const string &)
{
if (count ++)
{
count = 0; // reset count so only every second fails
throw runtime_error("Kaboom!");
}
cout << "Constructed\n";
}
~throwsecond()
{
cout << "Destructed\n";
}
};
int throwsecond::count = 0;
Then a simple class that does basically the above with less frills
class bad_example
{
throwsecond * a;
throwsecond * b;
public:
bad_example(): a(nullptr), b(nullptr)
{
}
bad_example (const string& a,
const string& b)
{
this->a = new throwsecond(a);
this->b = new throwsecond(b);
}
~bad_example()
{
delete a;
delete b;
}
};
and a main to exercise it
int main()
{
cout << "Bad example\n";
try
{
bad_example("", "");
}
catch (...)
{
cout << "Caught exception\n";
}
}
Output:
Bad example
Constructed
Caught exception
We have an object constructed and never destroyed.
Since a default constructor has been defined by Table we can, with a compiler that supports the C++11 or a more recent Standard, take advantage of delegated constructors to force destruction of the partially constructed object because it has been fully constructed by the default constructor.
class good_example
{
throwsecond * a;
throwsecond * b;
public:
good_example():
a(nullptr), b(nullptr) //must be nulled or destruction is dicey
{
}
good_example (const string& a,
const string& b) : good_example() // call default constructor
{
this->a = new throwsecond(a);
this->b = new throwsecond(b);
}
~good_example()
{
delete a;
delete b;
}
};
Output:
Good example
Constructed
Destructed
Caught exception
One construct and one destruct. The beauty of this approach is it scales well and adds nothing to the code that you don't already have. The cost is minimal, a and b get initialized and then assigned as opposed to just initialization. Faster code is useless if it doesn't work.
Full example: https://ideone.com/0ckSge
If you can't compile to a modern standard, you wind up doing something like
the next snippet to make sure everything is deleted. It's main sin is it's ugly, but as you add more classes that must be constructed and destroyed it starts getting unwieldy.
Table::Table(const string& tableID,
int numSeats,
const string& serverName):
tableID(NULL),
numSeats(numSeats),
serverName(NULL)
{
try
{
this->tableID(new string(tableID)),
// see all the this->es? don't shadow variables and you won't have this problem
// miss a this-> and you'll have a really bad day of debugging
this->serverName(new string(serverName))
// more here as required
}
catch (...)
{
delete this->tableID;
delete this->serverName;
// more here as required
throw;
}
}
There is probably a way to improve on this and make it more manageable, but I don't know it. I just use newer standards and value semantics (I'd love it if someone can provide a good link that describes this concept) where possible.
First i want to introduce my situation :
I have write some classes that has char* pointer as private class member.And also this project has GUI, so when click buttons,some functions may execute more than one time.Those classes are designed single class in project.But some functions of them can execute more than one time.Then I found my project has memory leak.
so i want to ask the following questions:
how to design the set function?
how to design the other functions that use the char* member variable?
how to design the class operator= function?
for example:
class A:
{
public :
setStr(char * s){//need new or just use =?};
A & operator=(const A& other){//also need new?};
manyTimesFunctions(char * other)
{
//need to use chars other to assignment str
//how to carefully use new to avoid memory leak?
//other may be another class's locality none const variable
}
private:
char * str;
}
So ,the project only init class A once,but may use setStr and manyTimesFunctions many times.
May be the answer:
I think i have found what i need to take care of:copy that class,that answers are really useful to me.
Just use std::string. It takes care of memory management for you. The member declaration then looks like
std::string str;
and the setter function looks like
void setStr( char const* s ) { str = s; }
Where you want to use the string and need a char const*, just write str.c_str().
With use of standard library types like std::string, and no manual dynamic allocation, you generally don't need to be concerned about operator=: the compiler-generated copy assignment works nicely.
By the way, it's generally a good idea to decide on some naming convention for member variables. Common ones for C++ include str_, mStr, and my_str. The underscore suffix is perhaps the most common one, but don't use a leading underscore like _str, because although technically allowed it conflicts with the conventions for implementation defined names (e.g. leading underscore is not allowed for identifiers in the global namespace).
I am not 100% sure what you are trying to do. However, since char* is a pointer you may be able to simply pass around the references.
char* operator=(char* s) { str = s; }
Just know that then if you modify value in your function it will modify the place you copied it from
If the char* needs to actually be a clone, so that it does not modify the original value. You first need to obtain the length of the char*.
This can be done with this function
unsigned Length(char* s)
{
unsigned I = 0;
while( *(s+I) != '\0')
I++;
return I;
}
The a new string can be created as follows
str = new char[LENGTH];
At that point you can copy the string over term by term
for(I = 0 ; I < LENGTH; I++)
{
str[I] = s[I];
}
Finally to avoid memory leaks this needs to be deleted in the class destructor
~A()
{
delete [] str;
}
Of course using std::string could save a lot of problems.
This answer will be used to contrast what the other answer(s) given that state to use std::string (and those answers are correct -- use std::string).
Let's assume that you could only use char *, you can't for some reason use std::string, and that you are dealing with NULL terminated strings. This is a synopsis of what your implementation would have to do (and please compare this with simply using std::string):
#include <algorithm>
#include <cstring>
class A
{
public:
// construct empty string
A () : str(new char[1]()) {}
// construct from non-empty
A(const char *s) : str(new char[strlen(s) + 1])
{ strcpy(str, s); }
// copy construct
A(const A& rhs) : str(new char[strlen(rhs.str) + 1])
{ strcpy(str, rhs.str); }
// destruct
~A() { delete [] str; }
// assign
A& operator=(const A& rhs)
{
A temp(rhs);
std::swap(str, temp.str);
return *this;
}
// setter
void setStr(char * s)
{
A temp(s);
*this = temp;
}
// getter
const char* getStr() { return str; }
private:
char * str;
};
Live Example
After adding a couple more constructors and a getter function, this follows the Rule of 3.
You see how much code we needed to add just to make the class safely copyable and assignable? That's why using std::string is much more convenient than using char * when it comes to class members. For std::string a single line needs to be changed, compared to adding the copy / assignment (and move, which I didn't show) functions.
The bottom line is that in C++ if you want strings, use strings (std::string) and try to keep away from using char * (unless you have a very compelling reason to be using char * to represent string data).
I have a very simple class named person which is given below , I have a problem with only two functions , i.e setstring () function and setname() function , I am calling setstring() function from the setname function.
The only problem is when in the main function I write
Object.setname(“Zia”);
The result is ok as shown in the output screen,
Now when I write
Object.setname(“Zia ur Rahman”);
Nothing is displayed as you can see the output screen.
I know the problem is when I pass the name pointer to setstring () function but I am confused about it please explain it in detail that what is happening here.
#include<iostream.h>
class person
{
char* name;
public:
person();
void setname(const char*);
void setstring(const char*, char*);
void print()const;
};
person::person()
{
name=new char[3];
strcpy(name,"NILL");
name[3]='\0';
}
void person::setstring(const char* s, char*p)
{
if(s!=NULL)
{
delete[] p;
p=new char[strlen(s)];
strcpy(p,s);
p[strlen(s)]='\0';
}
}
void person::setname(const char* n)
{
setstring(n, name);//passing data member name
}
void person::print()const
{
cout<<"Name: "<<name<<endl;
}
main()
{
person object;
object.setname("Zia ur Rahman");
object.print();
system("pause");
}
alt text http://img264.imageshack.us/img264/8867/inheritanceimage004.jpg
alt text http://img263.imageshack.us/img263/3770/inheritanceimage003.jpg
The specific reason that nothing is being printed is that in setstring, p is copy of the name pointer, not a reference to it. Try changing the signature of setstring to:
void setstring(const char* s, char*& p);
(note the &).
See the other answers for other significant errors in the code - unless these problems are fixed, you are likely to get crashes or strange behaviour.
And unless the purpose of the code is just to learn dynamic arrays, use std::string instead :-).
If you want to do correct OO-oriented programming in C++ you should maybe stay away from direct pointer management but use the STL string class and use references instead of pointers. Then you should have an easier time and your source should produce the correct output.
Otherwise check your constructor of the person class, the name-array has just 3 elements but you're referencing the 4th one there (indices 0-2!). Also in the setstring() method you're not allocating enough space for the trailing '\0' in the array!
For starters:
name=new char[3];
strcpy(name,"NILL");
name[3]='\0';
name has three elements - you are treating it as if it had five. In a similar vein:
p=new char[strlen(s)];
should be:
p=new char[strlen(s) + 1];
In fact, that function is completely wrong - you are supposed to be copying the string into 'name', not the mysterious 'p' parameter. If you really want that function, then 'p' must be a pointer to a pointer or a reference to a pointer.
Have you thought about using std::string from STL?
Though first problem I see is this.
person::person()
{
name=new char[3];
strcpy(name,"NILL");
name[3]='\0';
}
Your allocating an array of char's, the size of the array is 3 characters, then your copying "NILL" into it with strcpy so your filling in the array with all the characters but without the null terminator \0. "NILL" is a const string and such has a null terminator that is implicit but not shown for example "NILL\0". The \0 is a control character that is used to indicate the end of the string. Then you have an index out of bounds when you access the 3rd element of name array,when your size of your array is 3.
To help you find out the other parts that could be going wrong, here are some of the links to the function you use.
strcpy
strlen
strcpy will copy the whole string from one buffer to the next including the null terminator control character.
Strlen will return amount of characters between the beginning of the string and the terminating null character. Though it will not count the null character as a character. Thats why you will see suggestions with strlen(string)+1 to include the null terminator.
Though your doing well keep up the work, you will understand these little gotcha when using the standard library functions.
Suppose s have 5 chars. Then new char[strlen(s)]; allocates the memory of 5 chars. Then p[strlen(s)]='\0' is equivalent to p[5]=0. It's very very bad. p[5] does not exsist.
void person::setstring(const char* s, char*p)
{
if(s!=NULL)
{
delete[] p;
p=new char[strlen(s)];
strcpy(p,s);
p[strlen(s)]='\0'; //<<<< LOOK! Out of range
}
}
It's rather C, than C++ code.
There is C++ equivalent:
#include <string>
#include <iostream>
class person
{
std::string name_;
public:
person();
//preferable design is to set object invariant in appropriate constructor
explicit person(const std::string &name);
std::string get_name() const;
void set_name(const std::string &name);
void print()const;
};
person::person()
: name_("NILL")
{}
person::person(const std::string &name)
: name_(name)
{}
std::string person::get_name() const
{
return name_;
}
void person::set_name(const std::string &name)
{
name_ = name;
}
void person::print()const
{
std::cout<<"Name: "<<name_<<std::endl;
}
int main()
{
person person1;
person1.set_name("Zia ur Rahman");
person1.print();
//this is better design decision
//because after constructor we have valid object
person person2("Zia ur Rahman");
person2.print();
std::cin.get();
return 0;
}
//In header file: class definition:
class myString
{
public:
myString(void);
myString(const char *str);
myString(const myString &); //copy constructor
~myString(void); //destructor
void swap(myString &from);
private:
char *stringPtr;
int stringLen;
};
//in cpp file, defining them member functions
myString::myString(const char *str)
{
stringLen = strlen(str);
stringPtr = new char[stringLen+1];
strcpy(stringPtr,str);
cout << "constructor with parameter called"<<endl;
}
myString::myString(const myString &str)
{
stringPtr = new char[str.stringLen +1];
strcpy(stringPtr,str.stringPtr);
cout << "copyconstructor"<<endl;
}
void myString::swap(myString &from)
{
myString buffer(from);
int lengthBuffer = from.stringLen;
from = new char[stringLen+1];
from.stringLen = stringLen;
strcpy(from.stringPtr, stringPtr);
stringPtr = new char[lengthBuffer+1];
stringLen = lengthBuffer;
strcpy(stringPtr,buffer.stringPtr);
}
You can't modify a reference. Even if you replace it with a pointer modifying a pointer will not modify an object pointed to. Instead you need to work through the reference - just swap the fields.
void myString::swap(myString &from)
{
std::swap( stringLen, from.stringLen );
std::swap( stringPtr, from.stringPtr );
}
the above is using std::swap() as suggested by user sbi in comments. This is completely equivalent to the following (just for illustration, don't reinvent STL):
void myString::swap(myString &from)
// First remember own length and pointer
const int myOldLen = stringLen;
char* myOldPtr = stringPtr;
// now copy the length and pointer from that other string
stringLen = from.stringLen;
stringPtr = from.stringPtr;
// copy remembered length and pointer to that other string
from.StringLen = myOldLen;
from.StringPtr = myOldPtr;
// done swapping
}
Both will work even when called fro self-swapping:
myString string;
string.swap( string );
You have already gotten a few good answers concerning the errors in you myString::swap() function. Yet, I'd like to add another one. There's some many things wrong with that function, I first found it hard to think of where to begin. But then I realized that you fail on some fundamental issue which I'd like to point out:
As a convention, a function called swap is expected to perform its task
in O(1)
without ever throwing an exception.
(Yes, I know, there are exceptions: std::tr1::array<>::swap(). But those should be very well justified.) Your implementation fails on both accounts. It is O(n) (strcpy) and might throw an exception (new) -- and it does so unnecessarily and without justification.
When you look at myString, you'll see that it only has two pieces of member data, which both are of built-in type. That means swapping two objects of this class is really simple to do while keeping to the conventions mentioned above: just swap the member data. That's as simple as calling std::swap on them:
void myString::swap(myString &from)
{
std::swap(this->stringPtr,from.stringPtr);
std::swap(this->stringLen,from.stringLen);
}
This is will never fail (swapping two pointers and two integers cannot fail), executes in O(1), is very easy to understand (well, once you get a grip on that swapping, anyway; it is an idiomatic form of implementing a class-specific swap function), and consists of two lines of code calling something well-tested in the standard library instead of 8 lines of code doing error-prone (and, in your case, erroneous) manual memory management.
Note 1: Once you've done this, you should specialize std::swap to call your implementation for your class:
namespace std { // only allowed for specializing function templates in the std lib
template<>
inline void std::swap<myString>(myString& lhs, myString& rhs)
{
lhs.swap(rhs);
}
Note 2: The best (simple, exception-safe, and self-assignment-safe) way to implement assignment for your class is to use its swap:
myString& myString::operator=(const myString& rhs)
{
myString tmp(rhs); // invoke copy ctor
this->swap(tmp); // steal data from temp and leave it with our own old data
return *this;
} // tmp will automatically be destroyed and takes our old data with it
from = new char[stringLen+1]; should be from.stringPtr = new char[stringLen+1]; . Also remember to free the previously allocated memory before allocating new one.
Look closely at the line
from = new char[stringLen+1];
It is the same as
from = MyString(new char[stringLen+1]);
so your constructor of MyString get uninitialized array of chars. Then you trying to get the length of the string, but strlen just looping through chars of the string looking for 0 char. As we don't know what content uninitialized array of chars might have, we don't know what length strlen could return. It can even go further than array boundary and crash your program with segfault. But I can say for sure, after that there's not enough space in from.stringPtr to hold the string you want to copy in it.
So, use from.stringPtr = new char[stringLen+1]; or better from = MyString(*this); since you have copy constructor already.