std :: vector调整大小算法

Question

I'm actually trying to implement a simple copy of std::vector. During my tests to see if my data is consistent by comparing with the original one I noticed something.

std::vector<std::string> *v = new std::vector<std::string>(2);
std::string str1("Hello");
std::string str2("World");

v->push_back(str1);
v->push_back(str2);

v->resize(5);

for (std::vector<std::string>::const_iterator it = v->begin(); it != v->end(); it++)
{
    std::cout << "/" << (*it) << "/" << std::endl;
}

here is the result :

//
//
/Hello/
/World/
//

Can someone explain me why resize doesn't append std::string()'s like this :

/Hello/
/World/
//
//
//

What is the algorithm behind?

Answer 1

The key is here:

std::vector<std::string> *v = new std::vector<std::string>(2);

This creates a vector with 2 elements, which are default-constructed (meaning - two empty strings). Then you push_back Hello and World . Now you have 4 elements. Then the resize( 5 ) adds just one more element (also default-constructed).

I guess you wanted to manipulate/increase the capacity? You need std::vector::reserve instead.

---

You should create an empty vector and then use push_back s to see the behavior, you expect. Or just use operator[] instead of the two push_back s.

---

Is there a really good reason to create the vector on the heap, instead of on the stack? You (almost?) always should avoid this.

Answer 2

You are constructing a std::vector with 2 items inside, default constructed.
Reference: std::vector constructors

Replace:

std::vector<std::string> *v = new std::vector<std::string>(2);

with:

std::vector<std::string> *v = new std::vector<std::string>();

And, by the way, why are you allocating the vector on the heap with new? Just declare it as:

std::vector<std::string> v;

Answer 3

In the construction of vector you passed 2 as argument. If you take a look at that construction you will see this:

explicit vector(size_type _Count)

Here you are creating a vector with two default constructed string in it and then push two other string into vector. Use default construction and then use std::vector::reserve to increase the capacity of vector.

Answer 4

To get the result that you are wanting to see, try the following:

std::vector<std::string> *v = new std::vector<std::string>({"s1","s2"});

Then go about your resizing and looping. This should give you the expected padding as you have suggested.