This question already has answers here:
What is The Rule of Three?
(8 answers)
Closed 8 years ago.
I have problem with the following class. I think the problem is with string array, cause I made two other classes and the problem was the same. When I run the program it throws "double free or corruption", but I do not think any double corruption is possible. The problem is same with input string as reference or as common argument in Add method.
class WareH
{
public:
WareH(void)
{
first = true;
rows = 1;
inLine = 0;
cnt = 0;
max = 2;
cL = 0;
strs = new string[max];
}
~WareH(void)
{
delete [] strs;
}
bool Add(string& str, int ending)
{
if (first)
inLine++;
else
cL++;
if (ending == 0)
{
if (first)
first = false;
if (cL != inLine)
return false;
rows++;
}
strs[cnt++] = str;
Bigger();
return true;
}
void Bigger(void)
{
if(max == cnt)
{
max *= 2;
string* tmp = new string[max];
for (int i = 0; i < cnt; i++)
tmp[i] = strs[i];
delete [] strs;
strs = tmp;
}
}
friend ofstream& operator<<(ofstream& of,WareH war)
{
for (int a = 0; a < war.cnt; a++)
of << war.strs[a] << endl;
return of;
}
private:
bool first;
int rows, inLine, cnt, max, cL;
string* strs;
};
When a class manages resources, and releases them in its destructor, you must consider the Rule of Three to make sure that copying an object will not result in two objects managing the same resource.
That is what is happening here: the default copy constructor and copy-assignment operator will copy the pointer, giving you two objects which will both try to delete the same array on destruction. Solutions are:
Delete the copy constructor and copy-assignment operator to prevent copying; or
Implement them to copy the strings into a new array, not just the pointer; or
Use std::vector rather than messing around managing memory allocation yourself.
When I run the program it throws "double free or corruption", but I do not think any double corruption is possible.
Educated guess here:
The problem is not in the code you've shown, but in the client code. Here's what I think happens:
you wrote client code that instantiates (or assigns or returns by value or stores in a std container) WareH instances, and since you do not define a copy constructor and assignment operator (see "The Big Three"), they end up copying the values from your source objects. When the first of these instances (that are assigned to each other) are deleted, they delete the strs pointer.
When the second instance is deleted, they delete the same strs pointers that were deleted before (because the default copy constructors and assignment operators do not duplicate the allocated memory but just copy the pointers).
Solutions (if that is indeed, the problem):
working (and bad) solution: explicitly define copy construction and assignment operator for your class.
working (and good) solution: implement your strs as a std::vector<std::string> instead of std::string* and cnt.
Related
I am writing code for a backtracking approach to a Traveling Salesman type of problem. So at each point i will recurse for rest of the un-visited points.
I could not use any library/functions other than cout, cin, new and delete (so no vector). So for the problem i want to keep a track of what all points i have visited till now. I am using a dynamic boolean array for this. So i want to pass the dynamic array to a function as value to keep track of this.
This is what i have tried till now.
I tried to wrap the array in a struct, but the memory dealocation (delete) is giving error (Segmentation fault)
typedef struct Barray{
bool* a;
int size;
Barray(int size) { a = new bool[size]; this->size = size; }
Barray(const Barray& in) {
if(a) delete[] a; // error
a = new bool[in.size];
this->size = in.size;
for (int i = 0; i < in.size; i++)
a[i] = in.a[i];
}
~Barray() { delete[] a; } // error
}barray;
This is my recursive function call
void find_mindist(barray visited, int dist_now, int cur_p) {
if (base condition)
{return ;}
for (int i = 0; i < n; i++) {
if (visited.a[i]) continue;
barray tdist = visited;
tdist.a[i] = true;
int ndist = dist_now + dist(points[cur_p], points[i]);
find_mindist(tdist, ndist, i);
}
return ;
}
So my questions are -
how can i pass a dynamic array to a function as value?
Why is the delete above giving error?
First of all, the recommended approach for a local visited information is not the endless copying of the whole visited collection, but a mark->recurse->unmark approach. So whatever you do, please keep a single boolean array for the visited information and update its content to your needs.
The other problems occur because you try to delete an uninitialized pointer in the copy constructor. Also, the assignment operator should be overloaded as well to avoid unpleasent surprises. But non of this really matters if you don't copy your visited information anymore.
The problem this is a copy constructor. As such, on entry, a is uninitialized (so contains garbage), so the delete is invalid.
Barray(const Barray& in) {
if(a) delete[] a; // error
a = new bool[in.size];
this->size = in.size;
for (int i = 0; i < in.size; i++)
a[i] = in.a[i];
}
Just remove the delete line. Also, prefer to initialize members, rather
than assign them, so:
Barray(const Barray& in)
: a(new bool[in.size])
, size(in.size) {
for (int i = 0; i < in.size; i++)
a[i] = in.a[i];
}
Also, remember the Rule of Three. You need an copy assignment operator. The simplest is:
Barry& operator=(const Barray& in) = delete;
which just forces a compilation error if you try to use it! Better is:
Barry& operator=(const Barray in) { // **NOTE** pass by value!
std::swap(this.a, in.a);
std::swap(this.size, in.size);
}
This version provides the strong exception guarantee. You aren't allowed to use std::swap, so you'll either have to write your own, or write it out by hand (you choose).
Finally, if you ever find yourself returning a Barray, you should write a move constructor:
Barray(Barray &&in)
: a(in.a)
, size(in.size) {
in.a = nullptr;
}
This can save a lot of copying!
I tried to search same questions, but not one helped me. When I run program I get the "A Buffer Overrun has occurred..." error.
Constr:
Player(char* n)
{
length = strlen(n);
name = new char[length+1];
for(unsigned int i(0); i < length; i++)
name[i] = n[i];
name[length] = '\0';
}
Destr:
~Player(void)
{
delete [] name;
}
I've NULL terminated string and don't get out of bounds, what is problem?
There's no obvious error in the code you've posted, but trying to manage dynamic memory by juggling raw pointers will almost inevitably lead to errors like this.
Perhaps you haven't correctly implemented or deleted the copy constructor and copy-assignment operator, per the Rule of Three. In that case, copying a Player object will give two objects with pointers to the same array; both of them will try to delete that array, giving undefined behaviour.
The simplest solution is to manage your string with a class designed for managing strings. Change the type of name to std::string, and then the constructor can simply be something like
explicit Player(std::string const & n) : name(n) {}
and there's no need to declare a destructor (or move/copy constructor/assignment operators) at all.
So... a solution by using an std::string has been provided, but let me give another solution, keeping your member variables intact.
The problem is this. Suppose you have this code somewhere:
Player p1("Bob"); // Okay
Player p2("Annie"); // Okay
p2 = p1; // Oops! (1)
Player p3(p1); // Oops! (2)
At (1), the method Player& Player::operator=(const Player&) is called. Since you didn't provide one, the compiler generates one for you. When it does, it simply assumes that it may copy over all member variables. In this case, it copies over Player::name and Player::length. So, we have p1.name == p2.name. Now when the destructor of p2 is called, the allocated memory pointed to by p2.name is deleted. Then when the destructor of p1 is called, the same memory will be deleted (since p1.name == p2.name)! That's illegal.
To fix this, you can write an assignment operator yourself.
Player& Player::operator = (const Player& other)
{
// Are we the same object?
if (this == &other) return *this;
// Delete the memory. So call the destructor.
this->~Player();
// Make room for the new name.
length = other.length;
name = new char[length + 1];
// Copy it over.
for (unsigned int i = 0; i < length; ++i) name[i] = other.name[i];
name[length] = '\0';
// All done!
return *this;
}
At (2), the same problem occurs. You do not have a copy constructor, so the compiler generates one for you. It will also assume that it may copy over all the member variables, so when the destructors get called, they'll try to delete the same memory again. To fix this, also write a copy constructor:
Player::Player(const Player& other)
{
if (this == &other) return;
length = other.length;
name = new char[length + 1];
for (unsigned int i = 0; i < length; ++i) name[i] = other.name[i];
}
At the end of the day you should use an std::string though.
I'm having trouble with memory usage in my homework program, which is used to store information about companies and its' owners. Class СompanyTemplate represents this info.
public:
CompanyTemplate (const string & oName,
const string & oAddr,
const string & cName,
const string & cAddr);
Another Class CCompanyIndex is used to store multiple records using a dynamical array of pointers (I'm not allowed to use vectors). Here's CCompanyIndex constructor:
CCompanyIndex :: CCompanyIndex (void)
{
allocated = 1000;
current_size = 0;
pole = new CompanyTemplate* [allocated];
for (int i=0; i<allocated; i++)
{
pole[i] = NULL;
}
}
CCompanyIndex also provides methods Add (add record), Del (delete record), Search(search an information about owner's company.
I'm having trouble with the Add method, although all basic tests are good, I have memory leaks, as valgrind says, in Add method.
bool CCompanyIndex :: Add( const string & oName,
const string & oAddr,
const string & cName,
const string & cAddr )
{
int pos = findPos(oName, oAddr);
if(pos != -1)
{
return false;
}
if ((current_size)>=allocated)
{
CompanyTemplate ** temp;
allocated = allocated*2+1;
temp = new CompanyTemplate* [allocated];
for (int i=0; i<current_size; i++)
{
temp[i]=pole[i];
}
pole = temp;
for (int i=0; i<current_size; i++ )
{
if ((pole[i])->Compare(oName,oAddr)<0)
{
current_size++;
for (int k=current_size-1; k>=i; k--)
{
pole[i] = new Comp pole[k+1]=pole[k];
}anyTemplate(oName, oAddr, cName,cAddr);
return true;
}
}
pole[current_size] = new CompanyTemplate(oName, oAddr, cName,cAddr);
current_size++;
return true;
}
Array elements reallocating works as expected, more likely I have an error in destructor, but still can't find.
Here it is:
CCompanyIndex :: ~CCompanyIndex (void)
{
for (int i=0; i<allocated; i++)
{
delete pole[i];
}
delete [] pole;
pole = NULL;
}
Thanks
if ownership is unclear, just use std::shared_ptr.
of course in a professional setting a better answer might be to analyze better and get a better idea of ownership, like, is it really shared?
but absent that, use a std::shared_ptr.
by the way, it looks like your class fails to handle copying properly. this is called the "rule of three" (or for c++11, the "rule of five"). essentially, if you define either of destructor, copy constructor or copy assignment operator, then you most likely need all three in order to deal properly with copying.
but the simplest is to not define such operations but instead use standard library containers, like std::vector, and standard library smart pointers, like std::shared_ptr.
e.g. instead of defining pole as a raw pointer (to array), define it as a std::vector.
With such a general title, the general answer will: use a vector of shared_ptr.
But I assumed that your homework is to implement a kind of std::vector<CompanyTemplate> using "low" level C++, witout STL and smart pointers (with otherwise is the best way of using c++). So:
Maybe you have other error, but here are two:
CompanyTemplate ** temp;
allocated = allocated*2+1;
temp = new CompanyTemplate* [allocated];
int i=0
for (; i<current_size; i++)
{
temp[i]=pole[i];
}
for (; i<allocated ; i++) // you want to make NULL the new pointers
{
temp[i]=NULL
}
delete [] pole; // delete old array.
pole=temp;
Suppose we have the following:
class StringClass
{
public:
...
void someProcessing( );
...
StringClass& operator=(const StringClass& rtSide);
...
private:
char *a;//Dynamic array for characters in the string
int capacity;//size of dynamic array a
int length;//Number of characters in a
};
StringClass& StringClass::operator=(const StringClass& rtSide)
{
capacity = rtSide.capacity;
length = rtSide.length;
delete [] a;
a = new char[capacity];
for (int i = 0; i < length; i++)
a[i] = rtSide.a[i];
return *this;
}
My question is: why does this implementation of overloading the assignment operator cause problems when we try to assign an object to itself like:
StringClass s;
s = s;
The textbook I'm reading (Absolute C++) says that after delete [] a; "The pointer s.a is then undefined. The assignment operator has corrupted the object s and this run of the program is probably ruined."
Why has the operator corrupted s? If we're reinitalizing s.a right after we delete it, why does this cause such a problem in the program that we have to redefine the function as:
StringClass& StringClass::operator=(const StringClass& rtSide)
{
if (this == &rtSide)
//if the right side is the same as the left side
{
return *this;
}
else
{
capacity = rtSide.capacity;
length = rtSide.length;
delete [] a;
a = new char[capacity];
for (int i = 0; i < length; i++)
a[i] = rtSide.a[i];
return *this;
}
}
If you are assigning an object to itself both a and rt.a point to the same string, so when you do delete [] a you are deleting both what a and rt.a point to; then you do reallocate it, but the data you were going to copy (on itself) in the loop has been lost in the delete.
In the loop now you will just copy whatever junk happens to be in the memory returned by new on itself.
By the way, even with the "safety net" of the self-assignment check that assignment operator isn't completely ok (for instance, it's not exception safe); the "safe" way to define the "big three" (copy constructor, assignment operator, destructor) is using the "copy and swap idiom".
If you self-assign, you free (delete) the string via the LHS argument before you copy it to the newly allocated space via the RHS argument. This is not a recipe for happiness; it is undefined behaviour and anything may happen. A crash is plausible; if you're really unlucky, it may appear to work.
Consider what the value of rtSide.a is when you're inside the broken operator=.
It's the same as this->a, the values you just clobbered. Accessing non-owned memory is undefined behavior, thus accessing this->a is undefined behavior (since you just freed it).
delete [] a;
a = new char[capacity];
for (int i = 0; i < length; i++)
a[i] = rtSide.a[i]; //Invalid when this->a == rtSide.a
//because rtSide.a is no longer owned by your program.
If you did actually want to do this, you would have to make a copy of a before deleting it:
char* ca;
if (this == &rtSide) {
ca = copy of rtSide.a or this->a;
} else {
ca = rtSide.a;
}
//Do your assigning and whatnot
if (this == &rtSide) {
delete[] ca;
}
Obviously it's much more efficient to just do nothing instead of making temporary copies of all of an instances own members. It's the same concept as doing int x = 5; int y = x; x = y;
It is because you've first deleted the pointer delete [] a;
and then later on trying to copy from the deleted location:
for (int i = 0; i < length; i++)
a[i] = rtSide.a[i]; //rtSide has already been deleted as 'this' and '&rtSide' are same.
Remember it is the same location you are trying to copy from, which you've already deleted.
Hence, the error!
The later code you posted fixes this problem by checking for self-assignment as a separate case.
delete [] a;
a = new char[capacity];
for (int i = 0; i < length; i++)
a[i] = rtSide.a[i];
That's why. Think of it like this:
You delete whatever a points to, then allocate a new chunk of memory. The new chunk of memory contains garbage which becomes your new data. Do not be confused by the loop that does a[i] = rtSide.a[i]; that only copies the garbage onto itself.
Remember, this and rtSide both lead you to the same object. When you modify the object using this the object that rtSide refers to is modified.
I have these variables:
char** wordList_;
int wordListCapacity_;
int* wordCountList_;
char* fileName_;
int nUniqueWords_;
int nTotalWords_;
int nTotalCharacters_;
My copy constructor:
FileIndex::FileIndex(const FileIndex& fi)
{
fileName_ = new char[strlen(fi.fileName_) + 1];
strcpy(fileName_, fi.fileName_);
cout << "Jiasd?" << endl;
wordListCapacity_ = fi.wordListCapacity_;
nUniqueWords_ = fi.nUniqueWords_;
nTotalWords_ = fi.nTotalWords_;
nTotalCharacters_ = fi.nTotalCharacters_;
wordList_ = new char*[wordListCapacity_];
wordCountList_ = new int[wordListCapacity_];
for(int i = 0; i < nUniqueWords_; i++) {
wordList_[i] = fi.wordList_[i];
wordCountList_[i] = fi.wordCountList_[i];
}
}
My overloaded assignment operator:
FileIndex& FileIndex::operator=(const FileIndex& fi)
{
fileName_ = new char[strlen(fi.fileName_) + 1];
strcpy(fileName_, fi.fileName_);
wordListCapacity_ = fi.wordListCapacity_;
nUniqueWords_ = fi.nUniqueWords_;
nTotalWords_ = fi.nUniqueWords_;
nTotalCharacters_ = fi.nTotalCharacters_;
wordList_ = new char*[wordListCapacity_];
wordCountList_ = new int[wordListCapacity_];
for (int i = 0; i < nUniqueWords_; i++) {
wordList_[i] = new char[strlen(fi.wordList_[i])+1];
strcpy(wordList_[i], fi.wordList_[i]);
wordCountList_[i] = fi.wordCountList_[i];
}
return *this;
}
Whenever I create a FileIndex (called FirstIndex) and initialize the member variables with something meaningful (not NULL) I have these lines to test the copy constructor and assignment operator:
FileIndex secondIndex = firstIndex;
FileIndex thirdIndex;
secondIndex = thirdIndex; // Segmentation fault here
I get a segmentation fault with the assignment operator but I have a feeling it may be because of faulty code in the copy constructor. That being said, if there's an error in the copy constructor then there's also probably one in the assignment operator.
Thanks in advance for the help!
I think you want to use std::string and std::vector<T> for your class. Also, for the purpose of what goes wrong it would be necessary to see the default constructor and the destructor. From the look of you setup it seems that you may e.g. not have initialized some of the members in your default constructor. Also, you assignment operator has several resource leaks and will be pretty bad off if you try self assignment. In general, I'd recommend to implement assignment operators like this:
T& T::operator= (T other) {
other.swap(*this);
return *this;
}
This leverages the work done for the copy constructor and use a swap() member which is generally very easy to do.
Check out your copy constructor.
for(int i = 0; i < nUniqueWords_; i++) {
wordList_[i] = fi.wordList_[i];
wordCountList_[i] = fi.wordCountList_[i];
}
The problem is with wordList_[i] = fi.wordList_[i];. You are not allocating new memory and doing a strcpy here as you do in the assignment operator. Instead your new copy is actually pointing to data from the instance that it is copying from. I believe this may be what David Schwartz was alluding to.
It looks as if you may not initialize wordListCapacity_ correctly (it's hard to tell since you don't show the default ctor). Since it is an int, it can have a negative value, which can cause a segfault when you attempt wordList_ = new char*[wordListCapacity_];. There may be other problems.