C++继承 - 子类构造函数调用？

Question

我主要有以下内容:

Sum *sum = new Sum(Identifier("aNum1"), Identifier("aNum2"));

我的课程是:

class Table {
private:
    static map<string, int> m;    
public:
    static int lookup(string ident)
    {
        return m.find(ident)->second;
    }
    static void insert(string ident, int aValue)
    {
        m.insert(pair<string, int>(ident, aValue));
    }
};   

class Expression {
public:
    virtual int const getValue() = 0;
};

class Identifier : Expression {
private:
    string ident;
public:
    Identifier(string _ident) { ident = _ident; }
    int const getValue() { return Table::lookup(ident); }    
};

class BinaryExpression : public Expression {
protected:
    Expression *firstExp;
    Expression *secondExp;
public:
    BinaryExpression(Expression &_firstExp, Expression &_secondExp) {
        firstExp = &_firstExp;
        secondExp = &_secondExp;
    }
};

class Sum : BinaryExpression {
public:
    Sum(Expression &first, Expression &second) : BinaryExpression (first, second) {}
    int const getValue() 
    { 
        return firstExp->getValue() + secondExp->getValue();
    }
};

当我编译它时,我收到以下错误:

没有用于调用'Sum :: Sum(Identifier,Identifier)'的匹配函数

候选人是:Sum :: Sum(Expression&,Expression&)

Identifier类继承自Expression,为什么我会收到此错误？

Answer 1

问题是要传递的临时给你的构造函数,但构造函数需要一个非 - const参考,而临时对象只能绑定到const引用.

要修复它,请将参数类型更改为Expression const&.顺便说一下,这与继承和多态完全无关(但也需要digivampire的修复;我怀疑这只是一个错字).