C++ - Derive class off of std::string class to add extra function?

Question

First of all, I know that the std::string class has all the functions I could possibly need. This is just for testing purposes to see what I'd be able to do in the future.

Anyway, say I had this:

class MyString : public std::string { }

How would I then, for example, use:

MyString varName = "whatever";

because surely I'd get an error because "whatever" is a std::string not a member of the MyString class?

If you understand what I mean, how would I fix this?

(and by the way, I know this is probably a really silly question, but I am curious)

Mike Seymour · Accepted Answer · 2015-05-04 11:51:07Z

11

Deriving from a class simply to add member functions isn't a great idea; especially a non-polymorphic class like std::string. Instead, write non-member functions to do whatever you want with the existing string class.

If you really want to do this nonetheless, you can inherit the constructors:

class MyString : public std::string {
public:
    using std::string::string;
};

Now you can initialise your class in any way that you can initialise std::string, including conversion from a character array as in your example. (By the way, "whatever" isn't a std::string, it's a zero-terminated character array, const char[9].)

answered May 4, 2015 at 11:51

Mike Seymour

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8 Comments

Cory Kramer Over a year ago

Also regarding deriving from standard library classes in general, here are some things to consider.

Not a real meerkat Over a year ago

Maybe you should add a note that this only works in C++11. It's been a while since it was released, I know, but a beginner using gcc without -std=c++11 will still get a compiler error and be confused.

rozina Over a year ago

How does that work exactly. You can now write MyString str("hello") I assume. Can you also add new constructor overloads to the ones from std::string?

Mike Seymour Over a year ago

@CássioRenan: I'll leave such things to comments, if necessary. No beginner should be using obsolete dialects.

Mike Seymour Over a year ago

@rozina: This isn't really the place for a language tutorial. See en.cppreference.com/w/cpp/language/…, or your favourite C++ book. To answer your questions: yes and yes.

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