问一道题，，（c++ primer plus15章编程练习第4题）

#include <exception>
#include <stdexcept>
#include <string>
class Sales
{
public:
enum { MONTHS = 12 };
class bad_index : public std::logic_error
{
private:
int bi;
public:
explicit bad_index(int ix, const std::string & s = "Index error in Sales object\n");
int bi_val() const { return bi; }
virtual ~bad_index() throw () {}
};
explicit Sales(int yy = 0);
Sales(int yy, const double * gr, int n);
virtual ~Sales() {}
int Year() const { return year; }
virtual double operator[](int i)const;
virtual double & operator[](int i);
private:
double gross[MONTHS];
int year;
};
class LabeledSales : public Sales
{
public:
class nbad_index : public Sales::bad_index
{
private:
std::string lbl;
public:
nbad_index(const std::string & lb, int x,
const std::string & s = "Index error in LabeledSales object!\n");
const std::string & label_val() const { return lbl; }
virtual ~nbad_index() throw() {}
};
LabeledSales(const std::string & lb = "none", int yy = 0);
LabeledSales(const std::string & lb, int yy, const double * gr, int n);
virtual ~LabeledSales() {}
const std::string & Label() const { return label; }
virtual double operator[](int i) const;
virtual double & operator[](int i);
private:
std::string label;
};
#include "sales.h"
using std::string;
Sales::bad_index::bad_index(int ix, const string & s)
: std::logic_error(s) , bi(ix)
{
}
Sales::Sales(int yy)
{
year = yy;
for (int i = 0; i < MONTHS; ++i)
gross[i] = 0;
}
Sales::Sales(int yy, const double * gr, int n)
{
year = yy;
int lim = (n < MONTHS) ? n : MONTHS;
int i;
for (i = 0; i < lim; ++i)
gross[i] = gr[i];
for (; i < MONTHS; ++i)
gross[i] = 0;
}
double Sales::operator[](int i)const
{
if (i < 0 || i >= MONTHS)
throw bad_index(i);
return gross[i];
}
double & Sales::operator[](int i)
{
if (i < 0 || i >= MONTHS)
throw bad_index(i);
return gross[i];
}
LabeledSales::nbad_index::nbad_index(const std::string & lb, int ix,
const std::string & s) : Sales::bad_index(ix, s)
{
lbl = lb;
}
LabeledSales::LabeledSales(const std::string & lb, int yy)
: Sales(yy)
{
label = lb;
}
LabeledSales::LabeledSales(const std::string & lb, int yy, const double * gr, int n)
: Sales(yy, gr, n)
{
label = lb;
}
double LabeledSales::operator[](int i) const
{
if (i < 0 || i >= MONTHS)
throw nbad_index(Label(), i);
return Sales::operator[](i);
}
double & LabeledSales::operator[](int i)
{
if (i < 0 || i >= MONTHS)
throw nbad_index(Label(), i);
return Sales::operator[](i);
}
#include "sales.h"
#include <iostream>
using namespace std;
int main()
{
using std::cout;
using std::cin;
using std::cin;
double vals1[12] =
{
1220, 1100, 1122, 2212, 1232, 2334,
2884, 2393, 3302, 2922, 3002, 3544
};
double vals2[12] =
{
12, 11, 22, 21, 32, 43,
28, 29, 33, 29, 32, 35
};
Sales sales1(2011, vals1, 12);
LabeledSales sales2("Blogster", 2012, vals2, 12);
cout << "First try block:\n";
try
{
int i;
cout << "Year = " << sales1.Year() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales1[i] << ' ';
if (i % 6 == 5)
cout << endl;
}
cout << "Year = " << sales2.Year() << endl;
cout << "Label = " << sales2.Label() << endl;
for (i = 0; i <= 12; ++i)
{
cout << sales2[i] << ' ';
if (i % 6 == 5)
cout << endl;
}
}
catch (LabeledSales::nbad_index & bad)
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
catch (Sales::bad_index & bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "\nNext try block:\n";
try
{
sales1[2] = 37.5;
sales2[20] = 23345;
cout << "End of block 2:\n";
}
catch (LabeledSales::nbad_index & bad)
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
catch (Sales::bad_index & bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "bye!\n";
return 0;
}
这是原文件代码

要说是把上面的try后面的两个catch块改成只用一个catch块。。并且用上RTTI来确保合适调用lab_val（）。
我是这么改的。。
#include "sales.h"
#include <iostream>
#include <cstdlib>
#include <typeinfo>
using namespace std;
int main()
{
using std::cout;
using std::cin;
using std::cin;
double vals1[12] =
{
1220, 1100, 1122, 2212, 1232, 2334,
2884, 2393, 3302, 2922, 3002, 3544
};
double vals2[12] =
{
12, 11, 22, 21, 32, 43,
28, 29, 33, 29, 32, 35
};
Sales sales1(2011, vals1, 12);
LabeledSales sales2("Blogster", 2012, vals2, 12);
cout << "First try block:\n";
try
{
int i;
cout << "Year = " << sales1.Year() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales1[i] << ' ';
if (i % 6 == 5)
cout << endl;
}
cout << "Year = " << sales2.Year() << endl;
cout << "Label = " << sales2.Label() << endl;
for (i = 0; i <= 12; ++i)
{
cout << sales2[i] << ' ';
if (i % 6 == 5)
cout << endl;
}
}
catch (Sales::bad_index & bad)
{
if (typeid(bad) == typeid(LabeledSales::nbad_index))
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "\nNext try block:\n";
try
{
sales1[2] = 37.5;
sales2[20] = 23345;
cout << "End of block 2:\n";
}
catch (Sales::bad_index & bad)
{
if (typeid(bad) == typeid(LabeledSales::nbad_index))
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "bye!\n";
return 0;
}
但是编译器显示bad无法调用lable_val()方法。。。

但是将catch (Sales::bad_index & bad)改成catch (LabeledSales::nbad_index & bad)就可以编译通过并正确输出。。
我想知道catch (LabeledSales::nbad_index & bad) 为什么能传递LabeledSales::nbad_index的基类Sales::bad_index ?

问题有点长。。。见谅啊。。

。。。。其实主要问题就在4楼。。。直接先看看4楼就好。。

唉！没有人唉！。。。