变换算法给出 "binary '=' no operator which takes left-hand operand.."

Question

pair<CDrug, pair<unsigned,double>> expirednull(pair<CDrug,
pair<unsigned,double>> temp){
    if (temp.first.isValid() == false)
        temp.second.first = 0;
    return temp;
}

string checkForExpiredDrugs() {
    stringstream s;
    vector<CDealer>::iterator it1;
    map<CDrug, pair<unsigned, double>> d;
    map<CDrug, pair<unsigned, double>>::iterator it2;
    //transform algorithm
    for (it1 = this->m_dealers.begin(); it1 != this->m_dealers.end(); it1++) {
        s << "Dealer: " << it1->getCompany() << " " << it1->getRepresentative() << " " << it1->getTelephone() << endl;
        d = it1->getDrugs();
        transform(d.begin(),d.end(),d.begin(),expirednull);
        for (it2 = d.begin(); it2 != d.end(); it2++) {
            if (it2->first.isValid() == false) {
                it2->second.first = 0;
                s << "Expired: " << it2->first << endl;
            }
        }
        it1->setDrugs(d);
    }
    return s.str();
}

每当我运行程序时，它都会给我以下错误 ->

Error 7 error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const CDrug' (or there is no acceptable conversion)

Answer 1

这是因为地图元素实际上是： pair< const CDrug, ... > 不是 pair< CDrug, ... >

它们的键类型是常量，因为更改地图现有元素中的键会导致麻烦。（这会使元素未排序，从而破坏某些映射不变性）。

因此，您的转换函数返回的对象无法分配给地图元素 => 编译失败。

此外，您不能在地图上使用转换，因为您不能分配给地图迭代器（因为键是常量）。因此，您应该改用 for_each，如此处的相关问题所述：how to apply transform to a stl map in c++

类似于：

void expirednull(pair<const CDrug, pair<unsigned,double> > & temp)
{
    if( temp.first.isValid == false )
        temp.second.first = 0;
}

map< CDrug, pair<unsigned,double> > d;
for_each(d.begin(),d.end(),expirednull);

变换算法给出 "binary '=' no operator which takes left-hand operand.."

transform algorithm gives "binary '=' no operator which takes left-hand operand.."

c++

stdmap

assignment-operator