Using enum value as a entry in a map in c++

Question

I have an enum:

enum FlowType {

        LEVEL_FLOW = 1,
        PARTITION_FLOW = 3,
        ORDERBOOK_EVENT_FLOW = 4
}

I want to create a map such that on searching the integer part above, I must get the "LEVEL_FLOW" part back.

I am unable to come up with the map declaration and insertion statements. Please help.

This is waht I tried:

std::map<int, FlowType > FlowsMap;
FlowsMap.insert(std::make_pair<1, FlowType.LEVEL_FLOW >);

Why would you want to map an enum to an int with the same value? enums are ints. — Pat, Nov 29 '13 at 08:45
@Pat - what if the first one is an `id` of something and there are hundreds of `id`s and each of them has `FlowType` .. — Kiril Kirov, Nov 29 '13 at 08:45
@Pat: 0) He maps an `int` to an enum-type, not vice versa 1) enums are just as much `int` as pointers or `bool`s are — Sebastian Mach, Nov 29 '13 at 08:46

oleg_trofimov · Answer 1 · 2013-11-29T09:09:57.700

4

If you want to add values:

typedef enum e
{
    ONE = 1,
    TWO = 2,
    THREE = 3
} etype;

int main(int arc, char **argv)
{
    std::map <etype, std::string> mmap;
    mmap[THREE] = 3;
}

edited Nov 29 '13 at 09:09

answered Nov 29 '13 at 08:52

oleg_trofimov

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3

Do you know what's the value of `mmap[THREE]`? – Kiril Kirov Nov 29 '13 at 17:52

score 2 · Accepted Answer · answered Nov 29 '13 at 08:43

2

You should use just LEVEL_FLOW not FlowType.LEVEL_FLOW, if the enum is in the same scope. Otherwise, specify the scope, too.

answered Nov 29 '13 at 08:43

Kiril Kirov

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That's correct but you will still run into trouble trying to compile std::make_pair<1, LEVEL_FLOW >. – Julien B. Nov 29 '13 at 15:33
@JulienB. - like this: http://ideone.com/iDka7A ? What "trouble" do you have in mind? – Kiril Kirov Nov 29 '13 at 17:41

Julien B. · Answer 3 · 2013-11-29T15:35:21.957

The problem you are facing with insertion lay in your syntax. You should write :

FlowsMap.insert(std::make_pair(1, FlowType.LEVEL_FLOW));  
// make_pair is a function (make_pair()), pair is a type (pair <>)

or more nicely

FlowsMap[1] = LEVEL_FLOW;

There is no need to overload comparison operator for the value item in your map. Moreover, even if your key were to be the FlowType enum it would not be needed.

However I wonder if you really need a map for this... when you can cast your int in your enum type:

int myInt = 1;
FlowType ft = static_cast<FlowType> (myInt);

The only issue would be the case when some int has no enum value associated. You can prevent this with your map by using the find() method. If you know that case will not happen you can consider avoiding map use.

"or more nicely" - `insert` and `operator[]` are not (always) the same for `std::map`. — Kiril Kirov, Nov 29 '13 at 17:42

score 0 · Answer 4 · edited May 23 '17 at 11:51

0

Note that you are using C-style enums.

If you want to stick to them, you can fix it by changing your code to:

FlowsMap.insert(std::make_pair<1, LEVEL_FLOW >);

If you want the enum name to become part of the scope (like you are trying in your example), have a look at this thread on C++-style enums.

edited May 23 '17 at 11:51

Community

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answered Nov 29 '13 at 15:38

Domi

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Using enum value as a entry in a map in c++

4 Answers4