String类中的重载+运算符

Question

I have to implement a string class with operators [], +, +=, <<, >>, ==, !=, <, <=, >, >= but I cannot use the string object or include the library for c++. Here is my code so far:

# define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstring>
#include<fstream>
using namespace std;
class MyString {
public:
    MyString();
    MyString(const char* chars);
    int length() const;
    char * getValue()const;
    ~MyString();
    //copy constructor
    MyString(const MyString&);
    MyString& operator =(const MyString& s);
    char& operator[](int i);
    MyString& operator+=(const MyString& s);
    friend ostream& operator<<(ostream&os, const MyString&s);
    friend istream& operator >> (istream&is, MyString&s);

    friend bool operator ==(const MyString& s1, const MyString& s2);
    friend bool operator <(const MyString& s1, const MyString& s2);

private:
    char* value;
    int size;
};
MyString operator+(const MyString s1, const MyString& s2);
bool operator !=(const MyString& s1, const MyString& s2);
bool operator >=(const MyString& s1, const MyString& s2);
bool operator <=(const MyString& s1, const MyString& s2);
bool operator > (const MyString& s1, const MyString& s2);
//no arguments constructor
MyString::MyString() {
    value = 0;
    size = 0;
}
//constructor
MyString::MyString(const char* chars) {
    size = strlen(chars);
    value = new char[size + 1];
    strcpy(value, chars);
}
int MyString::length()const {
    return size;
}
char* MyString::getValue()const {
    return value;
}
MyString::~MyString() {
    delete[] value;
}
//copy constructor
MyString::MyString(const MyString& s) {
    size = s.size;
    value = new char[size + 1];
    strcpy(value, s.value);
}
MyString& MyString::operator=(const MyString&s) {
    if (s.value == 0) {
        delete[] value;
        value = 0;
        size = 0;
        return *this;
    }
    if (this != &s) {
        delete[] value;
        value = new char[s.size + 1];
        strcpy(value, s.value);
        size = s.size;
    }
    return *this;
}
char& MyString::operator[](int i) {
    if (i < 0 || i >= size) {
        cout << "Please enter correct value of index" << endl;
        exit(0);
    }
    else {

        return value[i];
    }
    return value[i];
}
MyString& MyString::operator+=(const MyString& s) {
    char** temp = &value;
    *temp = new char[size];
    strcpy(value,*temp);
    strcat(value, s.getValue());

    if (size != 0) {
        delete[] temp;
    }
    size = strlen(value);
    return *this;
}
ostream& operator<<(ostream&out, const MyString&s) {
    out << s.value;
    return out;
}
istream& operator >> (istream&in, MyString&s) {
    char*ptr = new char[100];
    in >> ptr;
    s = MyString(ptr);
    delete ptr;
    return in;
}
MyString operator+(const MyString s1, const MyString& s2) {
    MyString answer;
    answer += s1;
    answer+= s2;
    return answer;
}
bool operator ==(const MyString&s1, const MyString& s2) {
    return(strcmp(s1.value, s2.value) == 0);
}
bool operator<(const MyString& s1, const MyString& s2) {
    return (strcmp(s1.value, s2.value) < 0);
}
bool operator !=(const MyString& s1, const MyString& s2) {
    return !(s1 == s2);
}
bool operator>(const MyString& s1, const MyString& s2) {
    return !(s1 == s2) && !(s1 < s2);
}
bool operator<=(const MyString& s1, const MyString& s2) {
    return s1 < s2 || s1 == s2;
}
bool operator >=(const MyString & s1, const MyString& s2) {
    return !(s1 < s2);
}
void test_copy_and_destructor(MyString S) {
    cout << "test: copy constructor and destructor calls: " << endl;
    MyString temp = S;
    cout << "temp inside function test_copy_and_destructor: " << temp << endl;
}

int main() {

    MyString st1("abc abc");
    MyString st2("9fgth");

    cout << "Copy constructor , << operator" << endl;

    MyString  st3(st1);

    cout << "st3: " << st3 << endl;

    test_copy_and_destructor(st2);

    MyString  st4;

    cout << "operator + " << endl;

    st4 = st3 + st2;

    cout << "st4: " << st4 << endl;

    cout << "st1 + st2: " << (st1 + st2) << endl;

    cout << "operators  [ ] " << endl;

    for (int i = 0; i < st2.length(); i++)
    cout << st2[i] << " ";

    cout << endl;

    cout << "operators  += , ==, != " << endl;

    st2 += st1;

    if (st3 == st1)
    cout << "st3 and st1 are identical " << endl;
    else cout << "st3 and st1 are not identical " << endl;

    if (st2 != st1)
    cout << "st2 and st1 are not identical " << endl;
    else cout << "st2 and st1 are identical " << endl;

    cout << "operators  < , <=, >, >= " << endl;

    if (st2 < st1)
    cout << "st2 < st1 " << endl;
    else cout << "st2 is not less than st1 " << endl;

    if (st1 <= st2)
    cout << "st1 <= st2 " << endl;
    else cout << "st1 is not less than or equal to st2 " << endl;

    if (st1 > st2)
    cout << "st1 > st2 " << endl;
    else cout << "not (st1 >  st2) " << endl;

    if (st1 >= st2)
    cout << "st1 >= st2 " << endl;
    else cout << "not (st1 >=  st2) " << endl;

    cout << "operator >> " << endl;

    //Open the data file
    ifstream  input("A9_input.txt");
    if (input.fail()) {
        cout << "unable to open input file A9_input.txt, Exiting..... ";
    return 0;
    }
    MyString temp1;
    MyString temp2("aaa");
    input >> temp1;
    input >> temp2;
    cout << "first element of input file: " << temp1 << endl;
    cout << "second element of input file: " << temp2 << endl;
    input.close();

    cout << "MyString says farewell....." << endl;
    return 0;
}

After reaching operator + my code reaches break point and crashes. I could tell something is wrong with my += operator code as + is using += code. Can someone help me figure out what is wrong with my += code?

Answer 1

your problem is that you are trying to change what you pointer points to be using a pointer and not pointer to pointer. when you are using the new operator your pointer simply gets new adress at the heap and than the assignment you did before(char * temp = value) is irelvent. beacuse temp gets the adress pointed by value and not value adress, than any modification on temp adress will not effect value. what you really wanna do is :

char * temp = new char[size]
//modify temp(add the strings)
//change value to hold the modified string
value = temp;

you can also do that :

char ** temp = &value;//now temp holds value's address.
*temp = new char[size];
//modify temp will modify value, but you need to derefrence before each use.

update

you are trying to copy a larger string temp into value which casues overflow (its a good practise to use strcpy_s/strcat_s beacuse they cannot copy more than the length you give them, that should be the size of dest).

link to strcpy/strcpy_s doc : http://en.cppreference.com/w/c/string/byte/strcpy http://en.cppreference.com/w/c/string/byte/strcat

what you should do is :

char * temp = new char[size];
//strcpy_s(dest, length of dest, src)
strcpy_s(temp , sizeof temp, value);//copy value to temp
strcat_s(temp, sizeof temp, s.getValue());
if(value != nullptr)
  delete [] value;
value = temp;

Answer 2

The problem is that you are using strcpy() and strcat() to populate the old allocation, which is not large enough. Therefore, the strcat() invocation attempts to write past the end of this allocation and the result is undefined behavior, which is manifesting as a crash on your system.

char* temp = value;
temp = new char[strlen(value) + s.length() +1];
strcpy(value, temp);  // <-- should be copying the other way
strcat(value, s.getValue()); // <-- should be copying into temp, not value
//if (size != 0) {
//  delete[]temp;
//}
size = strlen(value);
return *this;

Rewritten to solve this problem, as well as release the original string, and check for value == nullptr :

char *oldString = value == nullptr ? "" : value;

int newSize = strlen(oldString) + s.length();
char *newString = new char[newSize + 1];

strcpy(newString, oldString);
strcat(newString, s.getValue());

delete [] value;

value = newString;
size = newSize;

return *this;