没有构造函数的类中的C ++非静态const成员

Question

I keep getting this error:

warning: non-static const member 'const char sict::Weather::_date [7]' in class without a constructor [-Wuninitialized]

And i don't understand it.

main.cpp

#include <iostream>
#include <iomanip>
#include "Weather.h"
using namespace std;
namespace sict{

int main(){
    int n; 
    Weather* weather;

    cout << "Weather Data\n";
    cout << "=====================" << endl;
    cout << "Days of Weather: ";
    cin >> n;
    cin.ignore();
    weather = new Weather(n);

    for (int i = 0; i < n; i++){
        char date_description[7];
        double high = 0.0, low = 0.0;

        cout << "Enter date: ";
        cin >> date_description;
        cout << "Enter high: ";
        cin >> high;
        cout << "Enter low : ";
        cin >> low;
    }
    cout << endl;
    cout << "Weather report:\n";
    cout << "======================" << endl;

    for (int i = 0; i < n; i++){
        weather[i].display();
    }

    delete[] weather;

    return 0;
}
}

Weather.cpp

#include <iostream>
#include <cstring>
#include <iomanip>
#include "Weather.h"

using namespace std;
namespace sict{

void weather::set(const char* date[6], double _high, double _low){
    strncpy(_date, date_description, 6);
    _high = high;
    _low = low;
}

void weather::display() const{
    cout << setw(6) << setfill('_') << left << setw(6) << _date << right << setw(6) << _high << _low << '_' << endl;
}
}

Weather.h

#ifndef SICT_WEATHER_H_
#define SICT_WEATHER_H_

namespace sict{
class Weather{
    const char _date[7];
    double _high;
    double _low;

public:
    void set(const char* _date[7], double _high, double _low);
    void display() const;

};
}
#endif

My compilation errors are as follows

Weather.h:10:21: warning: non-static const member ‘const char sict::Weather::_date [7]’ in class without a constructor [-Wuninitialized]
Weather.cpp:11:7: error: ‘weather’ has not been declared
Weather.cpp: In function ‘void sict::set(const char**, double, double)’:
Weather.cpp:12:11: error: ‘_date’ was not declared in this scope
Weather.cpp:12:18: error: ‘date_description’ was not declared in this scope
Weather.cpp:13:11: error: ‘high’ was not declared in this scope
Weather.cpp:14:10: error: ‘low’ was not declared in this scope
Weather.cpp: At global scope:
Weather.cpp:18:7: error: ‘weather’ has not been declared
Weather.cpp:18:26: error: non-member function ‘void sict::display()’ cannot have cv-qualifier
Weather.cpp: In function ‘void sict::display()’:
Weather.cpp:19:57: error: ‘_date’ was not declared in this scope
Weather.cpp:19:86: error: ‘_high’ was not declared in this scope
Weather.cpp:19:95: error: ‘_low’ was not declared in this scope
Weather.h:10:21: warning: non-static const member ‘const char sict::Weather::_date [7]’ in class without a constructor [-Wuninitialized]
In file included from w3_in_lab.cpp:15:0:
Weather.h:10:21: warning: non-static const member ‘const char sict::Weather::_date [7]’ in class without a constructor [-Wuninitialized]
w3_in_lab.cpp: In function ‘int sict::main()’:
w3_in_lab.cpp:29:27: error: no matching function for call to ‘sict::Weather::Weather(int&)’
w3_in_lab.cpp:29:27: note: candidates are:
Weather.h:8:8: note: sict::Weather::Weather()
Weather.h:8:8: note:   candidate expects 0 arguments, 1 provided
Weather.h:8:8: note: sict::Weather::Weather(const sict::Weather&)
Weather.h:8:8: note:   no known conversion for argument 1 from ‘int’ to ‘const sict::Weather&’

Answer 1

The compiler is telling you that you have a const member in your class. Being const , its value can only be set in the class constructor (and then never changed).

But the class doesn't have a constructor, so the value is never set.

Answer 2

You have a const member in your class Weather . In C++, non-static const members can only be initialized in the constructor initialization list (see this post ).

To solve this issue you can either:

• Declare the _data member as static , this way you can initialize it like this .
• Declare and define a constructor for your class Weather and initlialize _data in it.

Otherwise the member _data is never initialized, so using its value is Undefined Behavior .

If you are using at least C++11, you can also initialize non-static members in the class definition:

class Weather{
    const char _date[7] = "mydate";
}

Answer 3

• you should declare your class before definition. That's why you are getting compilation errors.
• and dont use const as you are updating it in your setter.