reserve() and capacity() for flat containers

I just finished the new chapter in "C++23 - The Complete Guide" about flat containers and would like to share and discuss my advice about how to use reserve() and capacity() for flat containers (thanks to Jonathan Wakely who was pointing parts of this out).
It might be a surprise that these member functions do not exist as usually vectors are used inside flat containers and reserve() is a key performance feature of them.

However, here is how you can reserve more memory:

For flat\_set and flat\_multiset:

​

auto vec = std::move(fset).extract(); // temporarily extract the underlying vector
data.reserve(vec.capacity() 5); // raise capacity by a factor of 5
fset.replace(std::move(vec)); // move the vector back into the flat set

For flat\_map and flat\_multimap:

​

auto newCapa = fmap.keys().capacity() 5; // raise capacity by a factor of 5
auto data = std::move(fmap).extract(); // extract underlying vectors
data.keys.reserve(newCapa); // raise capacity of vector for keys
data.values.reserve(newCapa); // raise capacity of vector for values
fmap.replace(std::move(data.keys), // move the vectors back
std::move(data.values))

Note also that there is another pretty hacky way, but only for flat maps and multimaps (here mapping strings to double's):

​

auto newCapa = fmap.keys().capacity() 5;
constcast<std::vector<std::string>&>(fmap.keys()).reserve(newCapa);
constcast<std::vector<double>&>(fmap.values()).reserve(newCapa);

Yes, ugly, but works... ;-)

I am still working on adding reserve() and capacity() to the standard flat containers (see wg21.link/p3779)



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