discards qualifiers

问题

I am not really sure what my problem is. it says "discards qualifier". I don't know why this happens. if I erase const for overloaded operator+= everything is fine. can someone help me out?

// practice on overloading operators on Time variables
#include <iostream>
using namespace std;

class Time {
 public:
  //constructor
  Time(const unsigned int& day = 0, const unsigned int& hour = 0,
       const unsigned int& minute = 0, const unsigned int& second = 0)
    : day_(day),
      hour_(hour),
      minute_(minute),
      second_(second) {
  }
  Time(const unsigned int& second = 0)
    : day_(0),
      hour_(0),
      minute_(0),
      second_(second) {
  }
  //copy constructor
  Time(const Time& time)
    : day_(time.day_),
      hour_(time.hour_),
      minute_(time.minute_),
      second_(time.second_) {
  }
  //assignment operator
  Time& operator=(const Time& time) {
    day_ = time.day_;
    hour_ = time.hour_;
    minute_ = time.minute_;
    second_ = time.second_;
    return *this;
  }
  //destructor
  ~Time() {} 
  //overloaded operators
  Time& operator+=(const Time& time); 
  Time operator+(const Time& time);
 private:
  unsigned int day_;
  unsigned int hour_;
  unsigned int minute_;
  unsigned int second_;
  void ConvertSecondsToTime();
  unsigned int TotalTimeInSeconds();
};
//main function
int main() {
  return 0;
}
  //overloaded operators
unsigned int Time::TotalTimeInSeconds() {
  return (day_ * 24 * 60 * 60 + 
          hour_ * 60 * 60 +
          minute_ * 60 +
          second_);
}
void Time::ConvertSecondsToTime() {
  while (second_ >= 60) {
    second_ -= 60;
    minute_ += 1;
  }
  while (minute_ >= 60) {
    minute_ -= 60;
    hour_ += 1;
  } 
  while (hour_ >= 24) {
    hour_ -= 24;
    day_ += 1;
  }
}
Time& Time::operator+=(const Time& time) {
  second_ = this->TotalTimeInSeconds() + time.TotalTimeInSeconds();
  ConvertSecondsToTime();
  return *this;
}
Time Time::operator+(const Time& time) {
  Time temp(*this);
  temp += time;
  return temp;
}
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my output is:

time_overloaded_operators.cpp: In member function ‘Time& Time::operator+=(const Time&)’:
time_overloaded_operators.cpp:75: error: passing ‘const Time’ as ‘this’ argument of ‘unsigned int Time::TotalTimeInSeconds()’ discards qualifiers

回答1:

The problem is that TotalTimeInSeconds is declared so as to forbid calling it via a const reference, but then operator+= tries to call it on time, which is a const reference.

The fix is to declare unsigned int TotalTimeInSeconds() const;, which says that the member function can be called via const references (and via const pointers, and the names of const objects).

回答2:

Change:

unsigned int TotalTimeInSeconds();

to

unsigned int TotalTimeInSeconds() const;

And therefore also:

unsigned int Time::TotalTimeInSeconds() {

to

unsigned int Time::TotalTimeInSeconds() const {

Which is basically saying that the Time object won't be modified by calling TotalTimeInSeconds, and so therefore const Time objects can be passed to operator+=. Otherwise it can't be sure that the Time object won't be modified when calling TotalTimeInSeconds inside of operator+=, and therefore the const qualifier is discarded. That's basically what the error means.

来源：https://stackoverflow.com/questions/8373010/discards-qualifiers