Question

I declare a variable string s;

and do s = "abc"; now it has buffer of 3 characters.

After

s = "abcd" it has a buffer of 4 characters.

Now after the third statement

s = "ab" question is will it keep the buffer of 4 characters or will it reallocate a 2 character buffer?

If it will allocate 2 character buffer is there any way I can tell it to keep the allocated maximum buffer.

So does it keep the buffer of maximum size ever allocated ?

s = "ab"
s="abc"
s="a"
s="abcd"
s="b"

Now it should keep a buffer of size 4.

Is that possible?

Answer 1

s = “ab” question is will it keep buffer of 4 words or will it
reallocate 2 word buffer ?

It will not reallocate the buffer. I don’t know if it’s mentioned in the standard, but all the implementations I have ever seen issue a reallocation only if they need to increase the capacity. Never to decrease. Even if you have a string with 4 characters and call .resize(2) or .reserve(2) the capacity will not change. In order to force the string (or containers) to reallocate memory to fit the exact size, there’s a simple swap trick for that

s.swap(string(s));

What happens here? You create a temporary from s which will have its capacity exactly equal to s.size() then swap it with your original string. The destructor of the temporary will free all the necessary resources.

Again, I am not claiming this is standard, but all the implementations I’ve seen have this behavior.