Inheritance C++ issue when assigning base class to sub class

This may be a silly question about inheritance, but it does not make much sense on why I am not allowed to do this. The base class I am inheriting fro开发者_JAVA百科m is std::string:

class A : public std::string

When I use class A, I want to do this:

A text;
std::string str = "hello";
text = str;

The compiler complains that there is no binary operator = to do this.

The reason I want to inherit from string is to add some extra functionality but not lost the functionality std::string gives by default.

Is there a reason why a base class cannot be automatically assigned to a sub class?

Thanks.

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You cannot inherit assignment operators, they are given special treatment. You can use using to bring them up from the base class.

Also, it's horrendously bad to inherit from Standard-provided classes, except iostream.

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Is there a reason why a base class cannot be automatically assigned to a sub class?

Yes. It is because the base class is not a subclass. You can only assign the other way around: assigning a subclass instance to a base class instance would be fine since the subclass is-a base class.

This is the general rule; the other answer explains what's specifically wrong in case of std::string.