I want to make a C++ enum of QString formats to display a QTime

Question

I've been working with C++ on a Time class in Qt and I need to write an enum class that contains the formats that I use in showTime(). Because of the nature of the QString formats I've been getting the following error, the value is not convertible to 'int'. I would gladly appreciate any help or suggestions.

P.S: Here's the code:

enum class TimeFormat {
    a = "hh:mm:ss",
    b = "hh:mm",
    c = "H:m:s a"
};

class Time {
public:
    Time();
    ~Time();

    void setTime(QTime t);
    QString showTime(){
         return time->toString(format);
    }
private:
    QTime* time;
    TimeFormat format;
};

enumeration underlying type must be an integer type. It can't be a string. You can associate string with an enumeration value using sone code. — Marek R, Sep 09 '21 at 17:12
See also [`How to Initialize a static const array of const strings (in C++) ? `](https://stackoverflow.com/a/6536848/8740349) — Top-Master, Sep 09 '21 at 17:37

Marek R · Answer 1 · 2021-09-10T07:05:03.687

2

enumeration underlying type must be an integer type. It can't be a string. You can associate string with an enumeration value using sone code.

Example (minimum C++17):

enum class TimeFormat {
    a,
    b,
    c
};

constexpr std::string_view toStringFormat(TimeFormat format)
{
    using namespace std::literals;
    constexpr std::array formats{ "hh:mm:ss"sv, "hh:mm"sv, "H:m:s a"sv};
    return formats[static_cast<size_t>(format)];
}

https://godbolt.org/z/Y6P6Wa4K5

edited Sep 10 '21 at 07:05

answered Sep 09 '21 at 17:16

Marek R

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1

Could we have `static` variables in `constexpr` function? – Ghasem Ramezani Sep 09 '21 at 17:22
1

yes static keyword is wrong, fixed. This happens when writing code without compiling. – Marek R Sep 10 '21 at 07:05

score 2 · Answer 2 · answered Sep 09 '21 at 17:19

The enumeration underlying type must be an integer type. So you couldn't use char const* for that. In your case, you could use a private array associated with your enumerations:

class Time {
public:
  enum class Format {
    a, b, c
  };

public:
    Time();
    ~Time();

    void setTime(QTime t);
    QString showTime(Format const f){
         time->toString(format.at(static_cast<size_t>(f));
    }
private:
    QTime* time;
    static inline std::array<char const*, 3> constexpr format = 
    { "hh:mm:ss", "hh:mm", "H:m:s a" };
};

Top-Master · Answer 3 · 2021-09-09T17:39:04.747

Enum's underlying type must be an integer in C++, but Swift and Java languages support something like what you tried.

Anyway, try something like below.

my-time.h file:

enum TimeFormat {
    a,
    b,
    c
};

class Time {
public:
    Time();
    ~Time();

    void setTime(QTime t);
    QString showTime();
private:
    QTime* time;
    TimeFormat format;
};

my-time.cpp file:

#include "my-time.h"

const char * const FORMAT_STRING_MAP[] = [
    "hh:mm:ss",
    "hh:mm",
    "H:m:s a"
];

QString Time::showTime()
{
    time->toString(FORMAT_STRING_MAP[format]);
}

score 0 · Accepted Answer · answered Sep 09 '21 at 19:33

I used a switch statement

enum class TimeFormat {
    format1,
    format2,
    format3,
};

class Time {
public:
    Time();
    ~Time();

    void setTime(int h, int m, int s);
    QString showTime() const{
        QString f;
        switch (format) {
            case TimeFormat::format1:
            f="hh:mm:ss";
            break;
         case TimeFormat::format2:
            f = "hh:mm";
            break;
         case TimeFormat::format3:
            f= "H:m:s a";
            break;
         }
         return time->toString(f);
    }

    void changeFormat(TimeFormat f);
private:
    QTime* time;
    TimeFormat format;
};

I want to make a C++ enum of QString formats to display a QTime

4 Answers4