两种重新初始化向量的方法有什么区别吗？

Question

I use two methods when I re-initialize vector

vector<int> vec(1000, 1);

//first
vec.assign(1000, 2);

//second
vec = vector<int>(1000, 3);

I think these two methods are produce same result, but I found that the second method takes up less memory.

Is there something difference between these two method??

Answer 1

The difference is how the internal memory of the std::vector is handled.

With

std::vector<int> vec(1000, 1);

You created a new std::vector called vec . It allocates memory to hold 1000 int s and initializes their values to 1 .

In the first method:

vec.assign(1000, 2);

You then tell vec to reuse that allocated memory, where the 1000 int s live. They are simply overwritten by values of 2 .

In the second method:

std::vec = std::vector<int>(1000, 3);

You actually do two things - you create a new std::vector which allocates memory for 1000 int s and initializes their values to 3 . Then this new vector is move assigned to vec , which in turn will throw away its memory of 2 s and take the internal memory of the anonymous unnamed vector with its 3 s, because its a so-called rvalue .

The overall memory consumption should be the same in the end, although when using the 2nd method, for a moment you have 2 allocated memory regions of 1000 int s each, alive at the same time.