CODEWITHSUNDEEP
c++c++11
Ascii Tome
Ascii Tome

Reputation: 1

create different type of object by parameter in c++?

I am new to c++, I want to create different type of object by parameter, like this:

if (version_ == "v1") {
    typename std::unordered_map<T, TIndex> uniq;
} else {
    typename absl::flat_hash_map<T, TIndex> uniq;
}

uniq.reserve(2 * N);

However, this code compile failed:

 error: 'uniq' was not declared in this scope
       uniq.reserve(2 * N);
       ^

Then, I change the code:

std::unordered_map<T, TIndex> uniq1;
absl::flat_hash_map<T, TIndex> uniq2;
auto uniq = uniq1;
if (version_ == "v2") {
   uniq = uniq2;
}
uniq.reserve(2 * N);

It also filed:

error: no match for 'operator=' (operand types are 'std::unordered_map<bool, long long int, std::hash<bool>, std::equal_to<bool>, std::allocator<std::pair<const bool, long long int> > >' and 'absl::flat_hash_map<bool, long long int, absl::hash_internal::Hash<bool>, std::equal_to<bool>, std::allocator<std::pair<const bool, long long int> > >')
           uniq = uniq2;
                ^

How can I implement this feature in c++?

Upvotes: 0

Views: 47

Answers (1)

molbdnilo
molbdnilo

Reputation: 66371

You can't determine a compile-time type at runtime.

What you can do is wrap all your work in templates, and then "kick off" with a specific type.
(That is, deploy the old "add a level of indirection" solution.)

Rough sketch:

template<typename Table>
void do_work(int N)
{
    Table t;
    t.reserve(2 * N);
    ....
}

//...
if (version == "v1")
{
    do_work<std::unordered_map<T, TIndex>>(N);
}
else
{
    do_work<absl::flat_hash_map<T, TIndex>>(N);
}

Upvotes: 1

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