c++ - 编译问题相等/不等运算符

Question

我有以下代码序列而且我不明白编译错误(代码下方)。

提前致谢

尤利安

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;
}

错误:

d:\Projects\cpptests>cl opequal.cpp Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86 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

您需要将 operator== 和 operator!= 函数声明为常量。

例如。

bool operator == (const X&r) const