Equality operator for QFlags with scoped enum

user_1548052 0
Viewed 503

I have some flags defined as follows, using a scoped enum:

enum class Capability : int
{
  NoCapabilities  = 0,
  SomethingCool = 1,
  UberCool = 1 << 1,
  EvenCooler = 1 << 2,
};
Q_DECLARE_FLAGS( Capabilities, Capability )

Now, I am trying to use the equality operator:

Capabilities(DataCapability::NoCapabilities) == Capability::NoCapabilities

I cannot use testFlag in the current example, since NoCapabilities = 0. This works if the enum is not scoped (removing class keyword).

Apparently, casting to int (static_cast<int>(NoCapabilities)) works, but it's really ugly.

What is the approach to solve this? Is it a bad practice to have a 0 zero value and test it?

flags c++11 enums qt
1 Answers

You can overload equality operator for your combination of data types. However cast ist still necessary but hidden by operator.

#include <QCoreApplication>
#include <QFlags>
#include <iostream>

enum class Capability : int
{
    NoCapabilities = 0,
    SomethingCool  = 1,
    UberCool       = (1 << 1),
    EvenCooler     = (1 << 2),
};
Q_DECLARE_FLAGS(Capabilities, Capability)

template <typename T>
bool operator==(const QFlags<T> lhs, const Capability rhs)
{
    return (QFlags<T>::Int(lhs) == static_cast<int>(rhs));
}

int main(int argc, char *argv[])
{
    Q_UNUSED(argc);
    Q_UNUSED(argv);

    if (Capabilities(Capability::NoCapabilities) == Capability::NoCapabilities)
    {
        std::cout << "true" << std::endl;
    }
    else
    {
        std::cout << "false" << std::endl;
    }

    if (Capabilities(Capability::EvenCooler) == Capability::NoCapabilities)
    {
        std::cout << "true" << std::endl;
    }
    else
    {
        std::cout << "false" << std::endl;
    }

    return 0;
}
user_115214820
Related