Question

I have the following code sequence And I don’t understand the compilation error (below the code).

Thanks in advance,

Iulian

class X {
public:

    int a;

    X()
    {
        a = 0;
    }

    bool operator == (const X&r)
    {
        return  a == r.a;
    }

    bool operator != (const X&r)
    {
        return !( *this == r );
    }
};

class DX : public X
{
public:
    int dx;
    DX()
    {
        dx = 1;
    }

    bool operator == (const DX&r)
    {

        if( dx != r.dx ) return false;
        const X * lhs = this;
        const X * rhs = &r;

        if ( *lhs != *rhs ) return false;

        return true;
    }

    bool operator != (const DX&r)
    {
        return !( *this == r );
    }
};

int main(void)
{
    DX d1;
    DX d2;
    d1 == d2;
    return 0;
}

The error:

d:\Projects\cpptests>cl opequal.cpp Microsoft (R) 32-bit C/C++
Optimizing Compiler Version 15.00.30729.01 for 80×86 Copyright (C)
Microsoft Corporation. All rights reserved.

opequal.cpp opequal.cpp(38) : error C2678: binary ‘!=’ : no operator
found which takes a lef t-hand operand of type ‘const X’ (or there is
no acceptable conversion)
opequal.cpp(16): could be ‘bool X::operator !=(const X &)’
while trying to match the argument list ‘(const X, const X)’

Answer 1

You need to declare your operator== and operator!= functions as const.

Eg.

bool operator == (const X&r) const