大括号的作用域是否用于注释目的，从而降低C++代码的速度

#include <iostream>
#include <utility>
#include <vector>
#include <string>
int main()
{
std::string str = "Hello";
std::vector<std::string> v;
{// uses the push_back(const T&) overload, which means 
// we'll incur the cost of copying str
v.push_back(str);
std::cout << "After copy, str is "" << str << ""n";
//other code involves local variable
}
{// uses the rvalue reference push_back(T&&) overload, 
// which means no strings will be copied; instead, the contents
// of str will be moved into the vector.  This is less
// expensive, but also means str might now be empty.
v.push_back(std::move(str));
std::cout << "After move, str is "" << str << ""n";
//other code involves local variable
}
std::cout << "The contents of the vector are "" << v[0]
<< "", "" << v[1] << ""n";
}