If you’re creating a lot of small vectors in C++, you should probably consider using a small-size optimized vector instead. But, if you can’t do that for some reason, you might wonder if there is any win to be had by reserve()ing a small size in advance.
While the standard doesn’t require vectors to have any particular initial capacity(), it seems like all the common implementations:
- Start with zero capacity
- Allocate a single element on the first
push_back()/emplace_back() - Grow to a pair of elements on the second push
- Grow at something vaguely resembling exponential on subsequent pushes
Here’s a Compiler Explorer sample to check this behavior on various compilers (adapted from metamorphosis’ answer on StackOverflow):
#include <iostream>
#include <vector>
int main()
{
std::vector<int> v;
std::cout << "Initial capacity: " << v.capacity() << "\n";
for(int i = 0; i != 10; ++i)
{
v.emplace_back(i); // use push_back() for pre-C++11 compilers, of course
std::cout << "Size " << v.size() << " capacity: " << v.capacity() << "\n";
}
return 0;
}GCC 5.1, GCC 10.2, Clang 6, and Clang 10.0.1 all grow as follows:
Initial capacity: 0 Size 1 capacity: 1 Size 2 capacity: 2 Size 3 capacity: 4 Size 4 capacity: 4 Size 5 capacity: 8 Size 6 capacity: 8 Size 7 capacity: 8 Size 8 capacity: 8 Size 9 capacity: 16 Size 10 capacity: 16
While MSVC 2013 and 2019 both have a much looser interpretation of exponential capacity growth:
Initial capacity: 0 Size 1 capacity: 1 Size 2 capacity: 2 Size 3 capacity: 3 Size 4 capacity: 4 Size 5 capacity: 6 Size 6 capacity: 6 Size 7 capacity: 9 Size 8 capacity: 9 Size 9 capacity: 9 Size 10 capacity: 13
