vector的reserve（），resize（），capacity（）

// 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;
}