I have a default constructor, copy constructor, destructor, assignment operator, length, and string functions. When I call the copy constructor and try to print the value of the new char*, I get a Read Access Violation _First was nullptr. The debugger shows iosfwd code and breaks at the error, but I have no clue what this means. Any help would be awesome.
This is where the debugger shows the error.
static size_t __CLRCALL_OR_CDECL length(const _Elem *_First)
{ // find length of null-terminated string
return (*_First == 0 ? 0 // <- this line
: _CSTD strlen(_First));
}
These are my functions (also not that we were not allowed to use STRCPY)
MyString::MyString() { //default constructor
string = new char[6];
int i = 0;
for (i; i < strlen(string); i++) {
string[i] = NULL;
}
string[i] = '\0';
}
MyString::MyString(const MyString &s) { //copy constructor
char* string = new char[strlen(s.string) + 1];
int i = 0;
for (i; i < strlen(s.string); i++) {
string[i] = s.string[i];
}
string[i] = '\0';
}
MyString::~MyString() { //destructor
delete[] string;
string = NULL;
}
MyString& MyString::operator=(const MyString& s) { //assignment operator
char* temp = new char[strlen(s.string) + 1];
int i = 0;
for (i; i < strlen(s.string); i++) {
temp[i] = s.string[i];
}
temp [i] = '\0';
delete[] string;
string = temp;
return *this;
}
size_t MyString::length() const { //length of string
return strlen(string);
}
char* MyString::cString() { //string
return string;
}
int main(int argc, const char * argv[]){
MyString s;
std::cout << "Empty: '" << s.cString() << "'\n";
s = "hello";
std::cout << "After operator=: '" << s.cString() << "'\n";
{
MyString t(s);
std::cout << "After copy: '" << t.cString() << "'\n";
s = t;
}
}
This line in your copy constructor
char* string = new char[strlen(s.string) + 1];
Creates a new local variable called string which means that your member variable called string is never assigned a value.
Remove the char *
Related
I've seen some similar questions before asking, but I'm still stuck at the part of concatenating two strings using operator+=.
Currently, I can get separate strings correctly by constructor method. But when I compile code, the line str[length+i] = s[i]; in the method String& String::operator+= (const String& s) shows an error:
no match for ‘operator[]’ (operand types are ‘const String’ and ‘unsigned int’)
So I need your help to fix this bug.
Here's my code:
#include <iostream>
#include <cstring>
class String {
// Initialise char array
char* data;
unsigned length;
public:
// Constructor without arguments
String();
// Constructor with 1 arguments
String(char* s);
// Copy Constructor
String(const String& source);
// Move Constructor
String(String&& source);
// Destructor
~String() { delete[] data; }
/*!
* #brief String length.
* #return Value in String #c length.
*/
unsigned len ( ) const;
/*!
* #brief Append to String.
* #param[in] s A String object.
* #return A String reference to *this.
* #post String will equal the concatenation of itself with #a s.
*/
String& operator+= (const String& s);
};
// Constructor with no arguments
String::String()
: data{ nullptr }
{
data = new char[1];
data[0] = '\0';
}
// Constructor with one arguments
String::String(char* s)
{
if (s == nullptr) {
data = new char[1];
data[0] = '\0';
}
else {
data = new char[strlen(s) + 1];
// Copy character of s[]
// using strcpy
strcpy(data, s);
data[strlen(s)] = '\0';
std::cout << data << "\n";
}
}
// Copy Constructor
String::String(const String& source)
{
data = new char[strlen(source.data) + 1];
strcpy(data, source.data);
data[strlen(source.data)] = '\0';
}
// Move Constructor
String::String(String&& source)
{
data = source.data;
source.data = nullptr;
}
unsigned String::len ( ) const
{
return length;
}
String& String::operator+= (const String& s)
{
unsigned len = length + s.len();
char* str = new char[len];
for (unsigned j=0; j < length; j++)
str[j] = data[j];
for (unsigned i=0; i < s.len(); i++)
str[length+i] = s[i];
delete data;
length = len;
data = str;
return *this;
}
int main()
{
// Constructor with no arguments
String a;
// Convert string literal to
// char array
char temp[] = "Hello world.";
// Constructor with one argument
std::cout << "s1: ";
String s1{ temp };
// Copy constructor
String s11{ a };
char temp1[] = "Goodbye!";
std::cout << "s2: ";
String s2{ temp1 };
String s3 = String s1 + String s2;
return 0;
}
Another way of writing main function:
int main()
{
String s1("Hello World.");
String s2("Goodbye!");
std::cout << "s1: " << s1 << std::endl;
std::cout << "s2: " << s2 << std::endl;
String s3 = s1 + s2;
std::cout << "s3: " << s3 << std::endl;
std::cout << "The last char of s3: " << s3[s3.size()-1] << std::endl;
return 0;
}
Expected result:
s1: Hello World.
s2: Goodbye!
s3: Hello World.Goodbye!
The last char of s3: !
How can I modify my code to get s3 and last char of s3 correctly?
In many of your constructors, you do not set length which leaves it with an indeterminate value - and reading such values makes the program have undefined behavior. So, first fix that:
#include <algorithm> // std::copy_n
// Constructor with no arguments
String::String() : data{new char[1]{'\0'}}, length{0} {}
// Constructor with one argument
String::String(const char* s) { // note: const char*
if (s == nullptr) {
data = new char[1]{'\0'};
length = 0;
} else {
length = std::strlen(s);
data = new char[length + 1];
std::copy_n(s, length + 1, data);
}
}
// Copy Constructor
String::String(const String& source) : data{new char[source.length + 1]},
length{source.length}
{
std::copy_n(source.data, length + 1, data);
}
// Move Constructor
String::String(String&& source) : String() {
std::swap(data, source.data);
std::swap(length, source.length);
}
In operator+= you are trying to use the subscript operator, String::operator[], but you haven't added such an operator so instead of s[i], use s.data[i]:
String& String::operator+=(const String& s) {
unsigned len = length + s.length;
char* str = new char[len + 1];
for (unsigned j = 0; j < length; j++) str[j] = data[j];
for (unsigned i = 0; i < s.length; i++) str[length + i] = s.data[i];
str[len] = '\0';
delete[] data; // note: delete[] - not delete
length = len;
data = str;
return *this;
}
If you want to be able to use the subscript operator on String objects, you would need to add a pair of member functions:
class String {
public:
char& operator[](size_t idx);
char operator[](size_t idx) const;
};
char& String::operator[](size_t idx) { return data[idx]; }
char String::operator[](size_t idx) const { return data[idx]; }
And for String s3 = s1 + s2; to work, you need a free operator+ overload:
String operator+(const String& lhs, const String& rhs) {
String rv(lhs);
rv += rhs;
return rv;
}
Also, to support printing a String like you try in your alternative main function, you need an operator<< overload. Example:
class String {
friend std::ostream& operator<<(std::ostream& os, const String& s) {
os.write(s.data, s.length);
return os;
}
};
Full demo
For starters neither constructor sets the data member length.
So the operator
String& String::operator+= (const String& s)
{
unsigned len = length + s.len();
char* str = new char[len];
//...
has undefined behavior.
Also provided that the data member length was initialized you need to write
char* str = new char[len + 1];
instead of
char* str = new char[len];
to reserve memory for the terminating zero character '\0' because you are using the standard C string function strcpy in the copy constructor
strcpy(data, source.data);
And the class does not have the subscript operator used in this for loo[
for (unsigned i=0; i < s.len(); i++)
str[length+i] = s[i];
And you forgot to append the terminating zero character '\0'.
Pay attention to that there is no member function size in the class used in this expression
s3[s3.size()-1]
And this construction
String s3 = String s1 + String s2;
is invalid. At least you should write
String s3 = s1 + s2;
and correspondingly define the operator +.
I have an assignment where I need to use an overloaded destructor to delete dynamically allocated pointers. However, when it runs, some of my pointers are deleted until a segmentation fault with one of my objects' pointers the one pointing to " and the second" made with a parameterized constructor.
I have tried to go through and make sure the delete operator had brackets (because my new operator did) I made sure the object still existed by printing out its information and address. I have tried to rewrite my allocation function, and I have tried to go over my destructor to see where it messes up.
If it helps, I have included my destructor, my allocation function, my deallocation function, and my parameterized constructor.
'''
//Destructor
MyString::~MyString()
{
buffer_deallocate();
};
void MyString::buffer_deallocate() {
cout << m_buffer << endl;
delete[](m_buffer);
m_buffer = NULL;
m_size = 0;
}
void MyString::buffer_allocate(size_t size) {
try {
m_buffer = new char[size];
m_size = size;
}
catch(bad_alloc&)
{
cout << "Errror: Unable to allocate memory" << endl;
buffer_deallocate();
}
}
//Parameterized Constructor
MyString::MyString(const char * str)
:m_size(0)
{
const char * strPtr = str;
while(*strPtr)
{
strPtr++;
m_size++;
}
buffer_allocate(m_size);
for(int i = 0; i < m_size; i++)
{
m_buffer[i] = str[i];
}
};
'''
Every time however I get the output after "and the second" to be
Segmentation fault (core dumped)
Edit: I have tried the majority of what was recommended. At least what I understood, the problem still persists, and I realize now I was kind of sparse on my code. (Please forgive me, I'm still learning.) Here is the new code along with the rest of the function file for reference:
'''
#include<iostream>
#include<string.h>
using namespace std;
#include"MyString.h"
//Default Constructor
MyString::MyString()
:m_size(0), m_buffer(NULL)
{
buffer_allocate(0);
};
//Parameterized Constructor
MyString::MyString(const char * str)
:m_size(strlen(str)+1), m_buffer(NULL)
{
buffer_allocate(m_size);
strncpy(m_buffer, str, m_size);
};
//Copy Constructor
MyString::MyString(const MyString & other)
:m_size(0), m_buffer(NULL)
{
const char * otherPtr = other.c_str();
buffer_allocate(other.size());
for(int i = 0; i < size(); i++)
{
m_buffer[i] = otherPtr[i];
}
m_buffer[m_size] = '\0';
};
//Destructor
MyString::~MyString()
{
buffer_deallocate();
};
size_t MyString::size() const
{
return m_size;
}
size_t MyString::length() const{
return m_size-1;
}
const char * MyString::c_str() const{
return m_buffer;
}
bool MyString::operator==(const MyString & other) const {
char * m_bufferPointer = m_buffer;
while(*m_bufferPointer++)
{
const char * str_ptr = other.c_str();
if(*m_buffer != *str_ptr++)
{
return 0;
}
}
return 1;
}
MyString & MyString::operator=(const MyString & rhs) {
buffer_deallocate();
buffer_allocate(rhs.size());
const char * c_strPtr = rhs.c_str();
int i;
for(i = 0; i < rhs.size(); i++)
{
this->m_buffer[i] = c_strPtr[i];
}
return *this;
}
MyString MyString::operator+ (const MyString & other_myStr) const {
char * temp_pointer;
temp_pointer;
size_t temp_size = m_size + other_myStr.size();
//New Combined Buffer for Concatanation
try {
temp_pointer = new char[temp_size];
temp_pointer = strcat(this->m_buffer, other_myStr.c_str());
}
catch(bad_alloc&)
{
cout << "Error: Unable to Allocate Memory";
return NULL;
}
return MyString(temp_pointer);
}
char & MyString:: operator[](size_t index) {
return m_buffer[index];
}
const char & MyString::operator[] (size_t index) const {
return m_buffer[index];
}
ostream & operator << (ostream& os, const MyString & myStr) {
os << myStr.m_buffer;
return os;
}
void MyString::buffer_deallocate() {
cout << "Trying to delete : " <<m_buffer << endl;
if(m_buffer){
delete[](m_buffer);
}
cout << " Success" <<endl;
m_buffer = NULL;
m_size = 0;
}
void MyString::buffer_allocate(size_t size) {
try {
m_buffer = new char[size];
m_size = size;
}
catch(bad_alloc&)
{
cout << "Errror: Unable to allocate memory" << endl;
m_size = 0;
}
}
'''
in MyString::buffer_deallocate
cout << m_buffer << endl;
requires that m_buffer be null-terminated. Unfortunately, MyString::MyString(const char * str) does not make this guarantee.
You could
for(int i = 0; i < m_size; i++)
{
cout << m_buffer[i] << endl;
}
to print the string out character by character instead, but it is probably more useful to waste a byte, null terminate, and take advantage of the Standard library
MyString::MyString(const char * str)
:m_size(0)
{
const char * strPtr = str;
while(*strPtr)
{
strPtr++;
m_size++;
}
buffer_allocate(m_size);
for(int i = 0; i < m_size; i++)
{
m_buffer[i] = str[i];
}
m_buffer[m_size] = '\0'; // add the null
}
and then
void MyString::buffer_allocate(size_t size) {
try {
m_buffer = new char[size+1]; // +1 for the null terminator
m_size = size;
}
catch(bad_alloc&) // this is a bad thing to do here. More on that later.
{
cout << "Errror: Unable to allocate memory" << endl;
buffer_deallocate();
}
}
But we can simplify this with a few library function calls.
MyString::MyString(const char * str)
:m_size(strlen(str))
{
buffer_allocate(m_size);
strcpy(m_buffer, str);
}
Addendum:
Your class may be violating the Rule of Three. If MyString does not have a copy constructor and an assignment operator to go along with the destructor any copies, deliberate or accidental, will turn a MyString into a time bomb. The destructor of one of the copies will be run before the others leaving the others without a valid m_buffer.
MyString::buffer_allocate cannot safely return void unless it allows the exception to propagate. Catching the bad_alloc leaves the object without a valid allocation at m_buffer and the rest of the program will not know this. Either every other access in the program must test for a valid buffer or engage in undefined behaviour trying to access the invalid memory. It's likely better to let the exception fall through and be caught by another part of the program that is better suited to making a decision on what to do.
MyString::buffer_allocate will leak an existing allocation if called on a MyString that already has a valid allocation at m_buffer. I recommend a
if (m_buffer)
{
delete[] m_buffer;
}
and initializing m_buffer to null in MyString's constructor
MyString(const char* str)
: m_size(std::strlen(str)), m_buffer(nullptr)
{
buffer_allocate(m_size);
std::strncpy(m_buffer, str, m_size);
}
So I had to remake this to some working code.
The problem might be that a string length must be extended with 1 to add the nul-termination. So that makes:
// prevent some MSVS warnings->errors
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string>
#include <cstring>
//Destructor
class MyString {
private:
size_t m_size;
char* m_buffer;
void buffer_allocate(size_t size) {
try {
m_buffer = new char[size];
m_size = size;
}
catch (std::bad_alloc&)
{
std::cout << "Errror: Unable to allocate memory\n";
// allocation failed, nothing changed, don't delete/
m_size = 0;
}
}
void buffer_deallocate() {
// printing in the buffer deallocation??
delete[] m_buffer;
m_buffer = nullptr;
m_size = 0;
}
public:
MyString(const char* str)
: m_size(std::strlen(str)+1) // add null termination
, m_buffer(nullptr)
{
buffer_allocate(m_size);
std::strncpy(m_buffer, str, m_size);
}
~MyString() {
buffer_deallocate();
}
void Print() const {
std::cout << m_buffer << '\n';
}
};
int main() {
std::string input{ "Hello World!" };
MyString myString(input.c_str());
myString.Print();
}
So I've overloaded an operator ">>" for my class MyString. It was supposed to read information from text file to a custom-made string, however I'm getting an exception when I'm trying to fill string char by char. Text.txt contains simply "xyz"
Main.cpp:
#include "MyString.h"
#include <fstream>
#include <iostream>
//TO-DO 1 CharArr for all
int main() {
MyString NewString;
ifstream In("Text.txt");
In >> NewString;
cout << NewString << endl;
system("pause");
return 0;
}
Constructors:
MyString::MyString() {
StringLength = 0;
Pointer = nullptr;
}
MyString::MyString(const char* String) {
for (int i = 0; String[i]; i++)
StringLength++;
Pointer = new char[StringLength + 1];
char *Source = (char *)String;
char *Destination = (char *)Pointer;
for (int i = 0; i < StringLength + 1; i++)
Destination[i] = Source[i];
}
Operator:
istream &operator>>(istream &In, MyString &String) {
int FileStringLength = 0;
char Character;
if (String.Pointer != nullptr)
delete[] String.Pointer;
while (In.get(Character) && Character != '\n')
FileStringLength++;
if (FileStringLength < 1000 && FileStringLength != 0) {
String.StringLength = FileStringLength;
In.clear(), In.seekg(0, ios::beg);
for (int i = 0; In.get(Character) && Character != '\n'; i++)
String.Pointer[i] = Character; // I get an exception here
}
else if (!FileStringLength) {
cout << "File is empty." << endl;
}
else {
cout << "File contains too many characters." << endl;
}
return In;
};
if (String.Pointer != nullptr)
delete[] String.Pointer;
freed the storage that
String.Pointer[i] = Character; // I get an exception here
was going to use.
Solution:
Don't delete[] the storage. Instead write over it until the storage is full, then allocate a new, larger buffer for storage, copy the old buffer into the new buffer, and then free the old buffer.
I created a user-defined String class and some file handling function. I tried using g++ on ubuntu to compile this piece of code and gdb to debug several times before. The error was thrown in line
s2.diskIn(iflile);
Interface:
class String {
long unsigned int len;
char *cStr;
public:
String(const String &);
explicit String(const char * const = NULL);
void diskIn(std::ifstream &);
void diskOut(std::ofstream &);
const char *getString();
String operator = (const String);
~String();
};
Copy constructor
String::String(const String &ss) {
if (ss.cStr == NULL) {
cStr = NULL; len = 0;
} else {
len = ss.len;
cStr = new char[len + 1];
strcpy(cStr, ss.cStr);
}
}
Default/ Assignment Constructor
String::String(const char * const p) {
if (p == NULL) {
cStr = NULL; len = 0;
} else {
len = strlen(p);
cStr = new char[len + 1];
strcpy(cStr, p);
}
}
Error function:
void String::diskIn(std::ifstream &fin) {
String temp;
fin.read((char *)&temp.len, sizeof(int));
temp.cStr = new char[len + 1];
int i;
for (i = 0; i < temp.len; i++)
fin.get(temp.cStr[i]);
temp.cStr[i] = '\0';
*this = temp;
}
Copy assignment operator
String String::operator = (const String ss) {
if (cStr != NULL) {
delete[] cStr;
cStr = NULL;
len = 0;
}
if (ss.cStr != 0) {
len = ss.len;
cStr = new char[len + 1];
strcpy(cStr, ss.cStr);
}
return *this;
}
Destructor
String::~String() {
delete[] cStr;
}
int main() {
String s2;
std::ifstream ifile("document.txt");
s2.diskIn(ifile); //Error was thrown here
std::cout << s2.getString() << "\n";
ifile.close();
return EXIT_SUCCESS;
}
I am new bee for c++ . I got follow error when I override operator+ .
ConsoleApplication1.out: /root/projects/ConsoleApplication1/sdstring.cpp:43: static sdstring::size_t sdstring::strlen(const char*): Assertion `str' failed.
This is my test code!
sdstring sd1(NULL);
cout << "sd1：" << sd1 << endl;
sdstring sd2("sd2");
cout << "sd2：" << sd2 << endl;
sdstring sd3;
cin >> sd3;
cout << "sd3：" << sd3 << endl;
sd3 +=sd2 ;
cout << "sd3：" << sd3 << endl;
sdstring sd4 =sd3+sd1;
cout << "sd4：" << sd2 << endl;
sd1 = sd2 + sd1;
cout << "sd1：" << sd1 << endl;
cout << "sd3==sd2:" << (sd3 == sd2)<<endl;
Error happened in This line .
sdstring sd4 =sd3+sd1;
This is my sdstring.cpp file.
#include "sdstring.h"
#include <assert.h>
sdstring::sdstring(const char *str) {
if (!str) {
datas = new char[1];
datas[0] = '\0';
}
else {
datas = new char[strlen(str)+1];
strcpy(datas, str);
}
}
sdstring::sdstring(const sdstring& str) {
datas = new char[strlen(str.datas) + 1];
strcpy(datas, str.datas);
}
sdstring& sdstring::operator+(const sdstring& str)const {
sdstring result(NULL);
size_t total_size = getlen() + str.getlen();
result.datas = new char[total_size + 1];
strcpy(result.datas, datas);
strcat(result.datas, str.datas);
return result;
}
bool sdstring::operator==(const sdstring& str)const {
return strcmp(datas, str.datas) == 0;
}
sdstring& sdstring::operator=(const sdstring& str) {
if (this == &str)
return *this;
delete[] datas;
datas = new char[str.getlen() + 1];
strcpy(datas, str.datas);
return *this;
}
sdstring::size_t sdstring::strlen(const char* str) {
assert(str);
size_t len = 0;
while ('\0' != *str++)
len++;
return len;
}
char* sdstring::strcpy( char* des, const char* src){
assert(des&& src);
char* temp = des;
while ('\0' != (*des++ = *src++));
return temp;
}
int sdstring::strcmp(const char* fir, const char* sec) {
assert(fir && sec);
while (*fir == *sec)
{
if (*fir == '\0') {
return 0;
}
++fir;
++sec;
}
return *fir - *sec;
}
char* sdstring::strcat(char* des,const char* src) {
char* temp = des;
while ('\0' != *des)
{
des++;
}
while ('\0' != (*des++ = *src++));
return temp;
}
sdstring::~sdstring()
{
if (datas)
{
delete[] datas;
datas = nullptr;
}
}
char& sdstring::operator[](const unsigned int position)const
{
return position < getlen() ? datas[position] : datas[position-1];
}
sdstring& sdstring::operator+=(const sdstring& str)
{
size_t total_size = getlen() + str.getlen();
if (total_size != getlen()) {
char* temp = datas;
datas = new char[total_size + 1];
strcpy(datas,temp);
strcat(datas, str.datas);
delete[] temp;
}
return *this;
}
ostream& operator<<(ostream& os, const sdstring& str)
{
os << str.datas;
return os;
}
istream& operator>>(istream& is, sdstring& str)
{
char* cache = new char[1024];
is >> cache;
delete[]str.datas;
str.datas = new char[sdstring::strlen(cache)];
sdstring::strcpy(str.datas, cache);
delete[]cache;
return is;
}
Who can help me , Thanks for first!
There are several things wrong with your code. However the operator + is wrong in that it is returning a reference to a local variable, which is undefined behavior.
sdstring& sdstring::operator+(const sdstring& str)const
{
sdstring result(NULL);
//..
return result; // Undefined behavior.
}
operator + should be returning a brand new object, not a reference to an object.
sdstring sdstring::operator+(const sdstring& str)const // <-- Note the return value is sdstring
{
sdstring result(NULL);
//..
return result;
}
The other issues with your code:
1) The operator= destroys the memory using delete[] datas; before calling new[] to allocate memory for the new string. If new[] throws an exception, the sdstring object would be corrupted since the data has been destroyed and you can't go back and reset the string with the old data. Use the copy / swap idiom to prevent this from happening.
2) You should store the length of the string in a member variable instead of calling strlen every time you want to know the length of the string. The strlen is slow in that it has to loop to count every single character to determine where the terminating '\0' character is located. Instead of that, just store the length of the string once and keep using that value.
3) Instead of writing your own strlen and strcmp functions, use the library functions strlen and strcmp. Your versions are not optimized, unlike the library versions.
4) operator + can be written in terms of operator +=. operator + should be written very simply as:
sdstring sdstring::operator + (const sdstring& rhs)
{
return sdstring(*this) += rhs;
}