public member function
<vector>
std::vector::reserve
void reserve (size_type n);
Request a change in capacity
Requests that the vector capacity be at least enough to contain n elements.
If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater).
In all other cases, the function call does not cause a reallocation and the vector capacity is not affected.
This function has no effect on the vector size and cannot alter its elements.
Parameters
- n
- Minimum capacity for the vector.
Note that the resulting vector capacity may be equal or greater than n.
Member type size_type is an unsigned integral type.
Return Value
none
If the size requested is greater than the maximum size (vector::max_size), a length_error exception is thrown.
If case of reallocation, the storage is allocated using the container's allocator, which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).
Example
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// vector::reserve
#include <iostream>
#include <vector>
int main ()
{
std::vector<int>::size_type sz;
std::vector<int> foo;
sz = foo.capacity();
std::cout << "making foo grow:\n";
for (int i=0; i<100; ++i) {
foo.push_back(i);
if (sz!=foo.capacity()) {
sz = foo.capacity();
std::cout << "capacity changed: " << sz << '\n';
}
}
std::vector<int> bar;
sz = bar.capacity();
bar.reserve(100); // this is the only difference with foo above
std::cout << "making bar grow:\n";
for (int i=0; i<100; ++i) {
bar.push_back(i);
if (sz!=bar.capacity()) {
sz = bar.capacity();
std::cout << "capacity changed: " << sz << '\n';
}
}
return 0;
}
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Possible output:
making foo grow:
capacity changed: 1
capacity changed: 2
capacity changed: 4
capacity changed: 8
capacity changed: 16
capacity changed: 32
capacity changed: 64
capacity changed: 128
making bar grow:
capacity changed: 100
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Complexity
If a reallocation happens, linear in vector size at most.
Iterator validity
If a reallocation happens, all iterators, pointers and references related to the container are invalidated.
Otherwise, they all keep referring to the same elements they were referring to before the call.
Data races
If a reallocation happens, the container and all its contained elements are modified.
Otherwise, the container is accessed, but not the contained elements: concurrently accessing or modifying them is safe.
Exception safety
If no reallocations happen or if the type of the elements has either a non-throwing move constructor or a copy constructor, there are no changes in the container in case of exception (strong guarantee).
Otherwise, the container is guaranteed to end in a valid state (basic guarantee).
The function throws length_error if n is greater than max_size.