Setting value to enum

Question

I'm a beginner in C++ so I'm sorry if this is super obvious, but I have an issue with assigning values to an enum. I've declared the enum like so in a header file:

 enum face
    {   paramControlHeight = 40,
        paramLabelWidth    = 80,
        paramSliderWidth   = 300
    };

And tried assigning an integer. Needless to say it doesn't work:

paramControlHeight = 40;//Not assignable

After googling about for a while, I tried:

using type_of_p=decltype(paramControlHeight);

Which as I understand, should yield the type of paramControlHeight, and enable me to use

paramControlHeight=static_cast<type_of_p> (40);

But I get the same "un-assignable" error

If anyone could point me in the right direction I'd be very grateful

What do you mean by "assign values to enum"? Do you want to create a variable of type face and assign value 40 to it? — Yksisarvinen
– Yksisarvinen, Commented Feb 27, 2020 at 20:52
I want to assign "paramControlHeight" which is inside my enum a different value. So, for example, it starts out as 40, but I would like to change it to 80 later on . — giannis101
– giannis101, Commented Feb 27, 2020 at 20:54
paramControlHeight is a constant. You assign variables with it: auto var = paramControlHeight; You can't assign new values to a constant. — Ted Lyngmo
– Ted Lyngmo, Commented Feb 27, 2020 at 20:55
That's not possible. Enums have set values. You need a variable (e.g. int paramControlHeight = 40) to be able to change it. — Yksisarvinen
– Yksisarvinen, Commented Feb 27, 2020 at 20:55

463035818_is_not_an_ai · Accepted Answer · 2020-02-27 21:33:28Z

I want to assign "paramControlHeight" which is inside my enum a different value. So, for example, it starts out as 40, but I would like to change it to 80 later on

You seem to misunderstand what enums are. You seem to expect the enum to behave like this

struct face
{   int paramControlHeight = 40;
    int paramLabelWidth    = 80;
    int paramSliderWidth   = 300;
};

face f;                     // create instance
f.paramControlHeight = 40;  // modify member

However, an enum is rather like a

struct face
{   
    static const int paramControlHeight = 40;
    static const int paramLabelWidth    = 80;
    static const int paramSliderWidth   = 300;
};

Now back to your actual enum:

enum face
{   paramControlHeight = 40,
    paramLabelWidth    = 80,
    paramSliderWidth   = 300
};

Here paramControlHeight is an enumerator with the value 40. You cannot modify it. It is not meant to be modified. It is meant to enumerate. What you can do is:

face f{ paramControlHeight };   // create instance of face
f = paramSliderWidth;           // assign a different value to it

A more typical enum would be

enum face_parts {
    nose = 1,
    eye = 2,
    mouth = 3
};

That you could use like this

void print_face_part( face_parts fp ){
    if (fp == nose) std::cout << "nose";
    if (fp == eye) std::cout << "eye";
    if (fp == mouth) std::cout << "mouth";
}

In simple terms an enum lets you name and group constants. Note that since C++11 there are scoped enums that are more flexible and dont introduce the name of the enumerators in the enclosing namespace.

Aykhan Hagverdili · Accepted Answer · 2020-02-27 21:14:56Z

1

paramControlHeight, paramLabelWidth, paramSliderWidth are the values. You can't assign anything to them more than you can assign a value to 42.

edited Feb 27, 2020 at 21:14

answered Feb 27, 2020 at 21:05

Aykhan Hagverdili

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