Different Between Capacity And Size in String

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here is a code :

C++

#include <iostream>
#include <string>
using namespace std;
int main() {
    std::string s1 = "Hello";
    cout << ".size() returns " << s1.size() << endl;
    cout << ".capacity() returns " << s1.capacity() << endl;
    s1.append("!");
    cout << ".size() returns " << s1.size() << endl;
    cout << ".capacity() returns " << s1.capacity() << endl;
    return 0;
}

This is an Output :

sometimes Size=6 and Capacity=12....
Why Its different?

What is different between Capacity and Size?

Thanks in advance.

Posted 29-Nov-10 3:18am

LetsTalk

Updated 29-Nov-10 10:35am

Emilio Garavaglia

v3

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#realJSOP · Answer 1 · 2010-11-29T03:24:00

Solution 1

Capacity is how many characters the string can hold, and size is the actual size of the string as it exists.

Posted 29-Nov-10 3:24am

#realJSOP

Comments

LetsTalk 29-Nov-10 9:30am

so how buffer know the size of capacity? how it calculate it since i didnt declear anythings?!

Richard MacCutchan · Answer 2 · 2010-11-29T07:50:00

Solution 2

Capacity is calculated when the string is created or resized, based on its current content. This allows a small increase in the string's contents without the necessity of allocating a new memory block. The actual value used is internal to the class.

Posted 29-Nov-10 7:50am

Richard MacCutchan

«_Superman_» · Answer 3 · 2010-11-29T08:06:00

All STL containers allocate memory dynamically based on demand.
But the memory allocated is always greater than what is actually required so that it cope with future additions to the container without allocating additional memory.

This is done because memory allocation is time consuming.
Also too much memory allocation will unnecessarily waste memory.

So there is a balance between the 2.

So size returns the actual size of the contents of the container and capacity returns the total memory currently allocated.

Try this -

std::string s1;
for (int i = 0; i < 100; ++i)
{
    s1.append("");
    cout << "Size = " << s1.size() << ", Capacity = " << s1.capacity() << endl;
}