How to use an operator from a specific base class on multiple inheritance context on C++

Question

I'm writing some C++ codes, and I can't compile the following code on g++. It only says that std::string hasn't a method named "operator==". I know it's not truth, but perhaps there is some multiple inheritance restrictions or problems I don't know yet.

The code:

#include<string>

struct Object{

    constexpr Object() noexcept = default;
    virtual ~Object() noexcept = default;

    virtual bool operator==( const Object& other) const noexcept = 0;
};


class String : public Object, public std::string{

    virtual ~String() noexcept = default;
    String() noexcept = default;

    virtual bool operator==( const Object& other) const noexcept{

        auto ptr =  dynamic_cast<const String*>(&other);

        return ptr != nullptr &&
            this->std::string::operator==(*ptr);    // here is the error
    }
};

int main(){}

The error:

$ g++ -std=c++11 test.cpp -o test.run

test.cpp: In member function ‘virtual bool String::operator==(const Object&) const’:
test.cpp:23:31: error: ‘class std::__cxx11::basic_string’ has no member named ‘operator==’; did you mean ‘operator=’? this->std::string::operator==(*ptr);

Answer 1

It doesn't have the operator as a member, it is a global operator.

((std::string&)*this) == (*ptr);

See non-member functions section in the docs: https://en.cppreference.com/w/cpp/string/basic_string