How to check two vectors of different types for equality [duplicate]

Question

I have two vectors, a, b, of structs abc and abc2 respectively:

struct abc {
    const int &data;
    abc(const int& cdata) : data(cdata) {}
};

struct abc2 {
    const int& data;
    abc2(const int& cdata) : data(cdata) {}

    bool operator== (const abc& c1) const {
        return (c1.data == data);
    }
};

int main() {

    std::vector<abc> a{abc(2), abc(2)};
    std::vector<abc2> b{abc2(2), abc2(2)};

}

And I want to see if a and b are equal.

However when I attempt the following:

assert(a == b);

I get a compile time error of

Error C2678 binary '==': no operator found which takes a left-hand operand of type 'std::vector<abc,std::allocator<_Ty>>' (or there is no acceptable conversion) with [ _Ty=abc ]

How can I implement a custom equality check for these two vectors of different types? I do not ever need to do abc == abc2 so a overload equals operator is not required for the abc struct.

The dupe seems to answer the question in your title. Is that what you're looking for, or are you trying to actually get == to work? — cigien
– cigien, Commented Nov 4, 2020 at 18:59

cigien · Accepted Answer · 2020-11-04 19:30:16Z

1

There is a standard algorithm for comparing equality: std::equal. In this case

assert(std::equal(b.begin(), b.end(), a.begin(), a.end()));

edited Nov 4, 2020 at 19:30

cigien

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answered Nov 4, 2020 at 18:36

eerorika

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2 Comments

Evg Over a year ago

You can use an overload of std::equal that takes four iterators instead of checking sizes explicitly.

eerorika Over a year ago

@Evg Oh nice, they added those in C++14.

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