c++ 矢量效率

Question

The standard requires that vector implementations can efficiently add elements at run time. Because vectors grow efficiently, it is often unnecessary—and can result in poorer performance—to define a vector of a specific size. The exception to this rule is if all the elements actually need the same value. If differing element values are needed, it is usually more efficient to define an empty vector and add elements as the values we need to become known at run time.

Reading in C++ prime (C++ 11 edition). Can someone explain to me why does that happens to be the case. I mean imagine we need to add somewhere 1 million different numbers into the std::vector. Why is that if I push_back them into the vector which hasn't defined approximate size will be the more "efficient" way than defining the size ? I mean I have a vague understanding of vector size and capacity and I believe every time Vector exceeds its capacity all the elements are copied into the bigger size array (n * 2 or n * 1.5 somewhere the neighborhood)...

Answer 1

For simple types such as ints, declaring a vector with the number of elements needed will likely be faster than using push_back , because the vector won't need to do any reallocations. However, with complex classes which have constructors, doing so will default-construct all the elements and then overwrite them, wasting time.

In both cases, the best solution would be to use reserve to allocate space for all the elements at once without constructing those elements. Then you can use push_back without worrying about reallocations.