Question

I am asked to create a Substring() method that returns a substring of the original string which begins at location start and is as long as length

Here is how I attempted to implement the function in my .cpp file:

MyString sub;

sub = new char[length];

for(int i = start; i <length; i++)
{
    sub[i] = this[i];
}

return sub;

and I got this error:

error: expected unqualified-id before [ token
MyString.cpp:206: error: no match for operator[] in sub[i]

Note: I am not supposed to overload [].
MyString is the defined class.

What exactly am I doing wrong?

Answer 1

It simply means that this.substr() is not valid C++.
this is a pointer to the current object. So unless the method MyString::substr() exists, you can’t do that.

Starting from that, I don’t know which members there are in your MyString class. If there is a std::string, you can use the substr method on it. f the underlying member is just a char*, you will have to use simple array and c-string operations on it to extract a subtring from it.

With an exemple. If your class is like this

class MyString
{
    private:
        std::string _str;

    public:
        // Constructor
        MyString(std::string str);
        // Your method
        MyString Substring(int start, int length) const;
};

Then your Substring method will be like this:

MyString MyString::Substring(int start, int length) const
{
    return MyString(_str.substr(start, length));
}

On the other hand, if your class MyString is like that:

class MyString
{
    private:
        char* _str;

    public:
        // Constructor
        MyString(char* str);
        // Your method
        MyString Substring(int start, int length) const;
};

Then your method will be like this:

MyString MyString::Substring(int start, int length) const
{
    char* res_str = new char[length+1];
    memcpy(res_str, (char*) _str + start, length);
    res_str[length] = '\0';
    return MyString(res_str);
}

EDIT : If we look at the code you provided (after last edit), it seems that you are actually using an underlying char*. So let’s have a look at what you wrote^^

MyString sub;
sub = new char[length];

What you want to do is actually modify the characters of the underlying char*. So what you should have done is:

char* sub;
sub = new char[length];

So instead of creating a new MyString, you will create a new char*. You can’t directly assign a char* to a MyString (or at least, it’s what I think). Now, let’s look a the other part of your code:

for(int i = start; i <length; i++)
{

        sub[i] = this[i];
}

this is a pointer to MyString. So this[i] is equivalent to this.operator which is incalid since this is a pointer. You can’t have this followed by a dot. However, if you had written this->operator, it would have searched for a function like char& MyString::operator[](int i). Since you did not defined this function, you would still have a compiler error (it’s also the one you currently have for sub[i] since you defined sub as a MyString. You should write:

for (int i = start; i < length; i++)
{
    sub[i] = _str[i];
}

But it’s still provided _str is a char* in your class. Then you will be able to finish your function by:

return MyString(sub);

But there, it’s provided that your MyString class has a constructor that takes a char* as a parameter 🙂