public member function
<unordered_map>
void reserve ( size_type n );
Request a capacity change
Sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements.
If n is greater than the current bucket_count multiplied by the max_load_factor, the container's bucket_count is increased and a rehash is forced.
If n is lower than that, the function may have no effect.
Parameters
- n
- The number of elements requested as minimum capacity.
Member type size_type is an unsigned integral type
Example
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// unordered_map::reserve
#include <iostream>
#include <string>
#include <unordered_map>
int main ()
{
std::unordered_map<std::string,std::string> mymap;
mymap.reserve(6);
mymap["house"] = "maison";
mymap["apple"] = "pomme";
mymap["tree"] = "arbre";
mymap["book"] = "livre";
mymap["door"] = "porte";
mymap["grapefruit"] = "pamplemousse";
for (auto& x: mymap) {
std::cout << x.first << ": " << x.second << std::endl;
}
return 0;
}
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Possible output:
book: livre
house: maison
grapefruit: pamplemousse
tree: arbre
apple: pomme
door: porte
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By calling reserve with the size we expected for the unordered_map container we avoided the multiple rehashes that the increases in container size could have produced and optimized the size of the hash table.
Iterator validity
If a rehash happens, all iterators are invalidated, but references and pointers to individual elements remain valid.
If no actual rehash happens, no changes.