Mapping enum value with templated function

Question

Lets say I have 10 templated functions, for example:

command1<T>(const std::string&) 
command10<T>(const std::string&, int timeInSeconds)

At a point in time in my code I will establish that I wish to execute a particular command. I will then request information regarding the type associated with this command (at runtime), which is returned to me via an enum. So I establish that I wish to execute command2 and the enum contains STRING. I therefore wish to call:

command2<std::string>(id, param1, param2);

What would you recommend as a good approach for doing this mapping?

The enum can contain INT, BOOL; DOUBLE or STRING. The arguments passed to a particular command are not dependant on the enum's value.

Example:

Here's an example to explain a little better:

Lets say my program receives "command4 a" from the command line. I parse this input and establish that I need to call command4. I then lookup the type associated with "a" and in this case get INT. I now need to call command4<int>("a");

What do you mean by "request information regarding the associated type"? Does this involve lookup at runtime? — Benjamin Bannier
– Benjamin Bannier, Commented Sep 18, 2012 at 11:29
If you only have that information available at runtime, you can only resolve this at runtime. Have you tried a lookup table containing all possible mappings? — Benjamin Bannier
– Benjamin Bannier, Commented Sep 18, 2012 at 11:41

ecatmur · Accepted Answer · 2012-09-18 12:09:55Z

2

A switch statement will work here:

switch (type) {
    case INT: commmand4<int>(id); break;
    ...
}

Depending on how you're actually calling the methods, templating on the arguments may be a good idea:

template<typename F, typename... Args> void call_function(Type type, Args... &&args) {
    switch (type) {
        case INT: return F::command<int>(std::forward(args)...);
        ...
    }
}

Note that because you can't pass a function template to a function, you'll have to wrap up the function templates inside a class:

struct Command1 {
    template<typename T> static void command(const std::string&);
};
...

call_function<Command1>(INT, "a");

edited Sep 18, 2012 at 12:09

answered Sep 18, 2012 at 11:54

ecatmur

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4 Comments

Baz Over a year ago

I have 10+ types and 10+ commands. I will have a lot of duplication since the arguments to the functions are not dependant on the type. But maybe this is the only way to do this...

ecatmur Over a year ago

@Baz you didn't mention that requirement. You can deal with that by templating on the argument types.

Baz Over a year ago

I don't understand. In my example, command4 takes a string argument but is called via command4<int>("a") since the enum contains an INT value. If the enum instead contained a BOOL, I would need to call command4<bool>("a") instead. The arguments are not effected.

Vaughn Cato Over a year ago

I think ecatmur is right, but it is probably a big conceptual leap for the OP. Ideally I think starting with a fully expanded example and then showing how to refactor with templates would be clearest, but it would be a long post.

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