相关疑难解决方法(0)

我正在尝试为另一个命名空间中定义的类型T定义一个等于运算符,然后使用相等运算符optional<T>.在clang(Apple LLVM 9.1.0)上,此代码:

    namespace nsp {
        struct Foo {
        };
    }
    bool operator==(const nsp::Foo& a, const nsp::Foo& b);

    void foo() {
        optional<nsp::Foo> a = none;
        optional<nsp::Foo> b = none;
        if (a == b)
            ;
    }

导致错误:

/usr/local/include/boost/optional/detail/optional_relops.hpp:29:34: error: invalid operands to binary expression ('const nsp::Foo' and 'const nsp::Foo')
{ return bool(x) && bool(y) ? *x == *y : bool(x) == bool(y); }
                      ~~ ^  ~~
MWE.cpp:40:19: note: in instantiation of function template specialization 'boost::operator==<what3words::engine::nsp::Foo>' requested here
            if (a …