【发布时间】：2017-10-23 18:48:56
【问题描述】：
我是 C++ 新手，我正在编写一个类来实现日期函数。
该程序有两个非成员函数bool printDate(const Date& d)和string intToString(const int& n)和两个用于重载运算符<< and >>的友元函数
我不断收到类似的错误
'std::__cxx11::string Date::month' is private within this context `,
` ambiguous overload for 'operator<<' (operand types are 'std::basic_ostream<char>' and 'const Date')`
and
`note: declared private here`
代码：
#include<iostream>
#include<vector>
#include<string.h>
#include<ctype.h>
#include<algorithm>
using namespace std;
const vector<string> months{ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
string intToString(const int& n){
return to_string(n); //inbuilt function
}
class Date{
//Private members
string month;
int day;
int year;
bool isValidMonth() const{
return day==daysInMonth(m);
}
int daysInMonth(const string& m) const{
vector<string>::iterator m_index;
if(find(months.begin(),months.end(),m)!=months.end()){
m_index=find(months.begin(),months.end(),m);
}
int month_index=distance(months.begin(),m_index)+1;
switch(month_index)
{
case 2: return isLeapYear()?29:28;
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
return 31;
default:
return 30;
}
}
bool isLeapYear() const{
if(year%4==0){
if(year%100==0 && year%400==0){ //2000 is leap year but 1900 is not
return true;
}
else{
return false;
}
}
else{
return false;
}
}
string toString(){
string result_date;
result_date=intToString(day)+"-"+month+"-"+intToString(year);
return result_date;
}
public:
friend istream& operator >>(istream& is, Date& d);
friend ostream& operator <<(ostream&os ,const Date);
//public constructor
Date(const string& m="January",const int& d=1,const int&y=2000){
month=m;
day=d;
year=y;
}
void setMonth(const string& m){
month=m;
}
void setDay(const int& d){
day=d;
}
void setYear(const int& y){
year=y;
}
string getMonth()const{
return month;
}
int getDay() const{
return day;
}
int getYear() const{
return year;
}
void Month(){
if(!month.empty())
{
month[0]=toupper( month[0] );
for(int i=1;i<month.length();++i)
month[i]=tolower(month[i]);
}
}
bool isValidDate() const{
//calls isValidMonth() to check if days match
if(isValidMonth() ){
return true;
}
return false;
}
};
istream & operator >>(istream & is, Date& d){
//overloads the >>operator, which reads
cout<<"\n Enter the day of the date \n";
cin>>d.day;
cout<<"\n Enter the month of the date \n";
cin>>d.month;
cout<<"\n Enter the year of the date \n";
cin>>d.year;
return is;
}
ostream & operator <<(ostream & os ,const Date& d){
os<<d.day<<"."<<d.month<<"."<<d.year;
return os;
}
bool printDate(const Date& d){
// if(!d.isValidDate()){
if(!d.isValidDate()){
cerr<<"( "<<d<<" ): not valid date";
return false;
}
cout<<"( "<<d<<" )";
return true;
}
int main(int argc, char const *argv[])
{
// code for testing purposes
Date defaultDate;
printDate(defaultDate);
cout<<endl;
/*
Date moonLanding("jul",20,1969);
printDate(moonLanding);
cout<<endl;
moonLanding.setMonth("july");
printDate(moonLanding);
cout<<endl;
Date leapDay("Ferbruary",29,2001);
printDate(leapDay);
cout<<endl;
leapDay.setDay(28);
printDate(leapDay);
cout<<endl;
Date d;
cout<<d.getMonth()<<" "<<d.getDay()<<", "<<d.getYear<<endl;
d.setMonth("July");
d.setDay(4);
d.setYear(1776);
printDate(d);
cout<<endl;
while(cin>>d){
d.Month();
bool flag=printDate(d);
cout<<endl;
if(flag) cout<<d.toString()<<endl;
}
*/
return 0;
}
程序运行时的错误是：
main.cpp: In function 'std::ostream& operator<<(std::ostream&, const Date&)':
main.cpp:133:17: error: 'int Date::day' is private within this context
os<<d.day<<"."<<d.month<<"."<<d.year;
^~~
main.cpp:19:14: note: declared private here
int day;
^~~
main.cpp:133:29: error: 'std::__cxx11::string Date::month' is private within this context
os<<d.day<<"."<<d.month<<"."<<d.year;
^~~~~
main.cpp:18:17: note: declared private here
string month;
^~~~~
main.cpp:133:43: error: 'int Date::year' is private within this context
os<<d.day<<"."<<d.month<<"."<<d.year;
^~~~
main.cpp:20:14: note: declared private here
int year;
^~~~
main.cpp: In function 'bool printDate(const Date&)':
main.cpp:139:23: error: ambiguous overload for 'operator<<' (operand types are 'std::basic_ostream<char>' and 'const Date')
cerr<<"( "<<d<<" ): not valid date";
~~~~~~~~~~^~~
main.cpp:132:19: note: candidate: std::ostream& operator<<(std::ostream&, const Date&)
ostream & operator <<(ostream & os ,const Date& d){
【问题讨论】：
-
"该类有两个非成员函数" - 任何类都只能有成员函数。:)
-
如果您唯一的问题是
>>运算符，那么转储大量代码的目的到底是什么，其中包含与当前问题无关的所有成员函数？ -
错误信息有什么不清楚的地方？ 1) 你试图在
ostream & operator <<(ostream & os ,const Date& d)中使用私人成员，而它没有被标记为朋友。 2) 任何可以用const Date&调用的东西，也可以用const Date调用，反之亦然。因此 - 模棱两可的调用错误。 -
提取运算符
>>不应与用户交互。它应该从istream参数中读取整个日期，而不是从cin，最好采用插入运算符<<编写的格式。
标签： c++ oop c++11 operator-overloading