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copied!<p>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.</p> <p>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.</p> <p>With an exemple. If your class is like this</p> <pre><code>class MyString { private: std::string _str; public: // Constructor MyString(std::string str); // Your method MyString Substring(int start, int length) const; }; </code></pre> <p>Then your Substring method will be like this:</p> <pre><code>MyString MyString::Substring(int start, int length) const { return MyString(_str.substr(start, length)); } </code></pre> <p>On the other hand, if your class MyString is like that:</p> <pre><code>class MyString { private: char* _str; public: // Constructor MyString(char* str); // Your method MyString Substring(int start, int length) const; }; </code></pre> <p>Then your method will be like this:</p> <pre><code>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); } </code></pre> <p>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^^</p> <pre><code>MyString sub; sub = new char[length]; </code></pre> <p>What you want to do is actually modify the characters of the underlying char*. So what you should have done is:</p> <pre><code>char* sub; sub = new char[length]; </code></pre> <p>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:</p> <pre><code>for(int i = start; i <length; i++) { sub[i] = this[i]; } </code></pre> <p>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:</p> <pre><code>for (int i = start; i < length; i++) { sub[i] = _str[i]; } </code></pre> <p>But it's still provided _str is a char* in your class. Then you will be able to finish your function by:</p> <pre><code>return MyString(sub); </code></pre> <p>But there, it's provided that your MyString class has a constructor that takes a char* as a parameter :)</p>

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