std::string::length, std::string::capacity, std::string::size in C++ STL

Prerequisite: String in C++

String class is one of the features provided by the Standard template library to us, So it comes up with great functionality associated with it. With these Functionalities, we can perform many tasks easily. Let's see a few of the functionalities string class provides.

Header File <string>

String Functionalities

There are many functionalities associated with string class but we will be focusing on only three of them, these are: 

• std :: string :: length 
• std :: string :: size
• std :: string :: capacity
• std :: string :: reserve()
• std :: string :: shrink_to_fit()

All three functions are public member functions of the Class String. To know more about other functionalities in string class refer to a String in STL.

1. std::string::length

Returns the length of a string. This returns the exact no. of characters or bytes used and not the memory that is allocated at the backend. length member function never throws exceptions.

Syntax:

size_t std::string::length( )

Note: 

1. size_t is a typedef version of unsigned int, as sizes cannot be negative.
2. No parameters are passed, the string is identified with this pointer.

2. std::string::size

Returns the length of the string, in terms of bytes. This returns the exact no. of characters or bytes used and not the memory that is allocated at the backend. Both length and size are the same and return the same value but size can be used in any container like vector, list, etc whereas length is more associated with the string. size member function never throws exceptions.

Syntax:

size_t std::string::size( )

3. std::string::capacity

• Returns the size of the storage space currently allocated in the Memory for that string object.
• This does NOT return the exact no. of characters or bytes used but, the memory that is allocated at the backend.
• Capacity is always GREATER THAN EQUAL TO size/length.
  • The extra space allows the string object to optimize its operations when new characters are added.
• Capacity is NOT limited, as needed the capacity is increased by the compiler.
• Capacity does not Reduce automatically, it needs to be done explicitly.

Syntax:

size_t std::string::capacity( );

Code:

// C++ Program to demonstrate use of
// std::string::capacity
// std::string::size
// std::string::length
#include <iostream>
#include <string>

using namespace std;

int main()
{
    // empty str
    string str;
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // H - only one character
    str = "H";
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // Hello
    str = "Hello";
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // Hello with space added
    str = "Hello ";
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // one more modification
    str = "Hello GFG Reader";
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // Back to Hello
    str = "Hello";
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // reserve is basically request to compiler to change
    // the capacity decrease capacity
    str.reserve(7);
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    // increase capacity
    str.reserve(35);
    cout << "str is : " << str << "\n";
    cout << "size: " << str.size()
         << " length: " << str.length()
         << " capacity: " << str.capacity() << "\n";

    return 0;
}

str is : 
size: 0 length: 0 capacity: 0
str is : H
size: 1 length: 1 capacity: 1
str is : Hello
size: 5 length: 5 capacity: 5
str is : Hello 
size: 6 length: 6 capacity: 10
str is : Hello GFG Reader
size: 16 length: 16 capacity: 20
str is : Hello
size: 5 length: 5 capacity: 20
str is : Hello
size: 5 length: 5 capacity: 7
str is : Hello
size: 5 length: 5 capacity: 35

The above code demonstrates all the possible scenarios. The facts have also proven that Capacity >= size/length, then the capacity increases automatically as the size of the string crosses the previous capacity, then the capacity is never reduced back when the size goes down, but that can be done explicitly by the programmer using reserve function.

Time and Space Complexity

4. std::string::reserve

• Reserves a specified amount of storage space in memory for the string object without changing its length or content. 
• This function can be used to avoid multiple memory reallocations when appending characters to a string.
• The function reserves the memory to accommodate at least the specified number of characters or bytes, but the actual capacity may be greater than that. 
• If the requested capacity is less than or equal to the current capacity, the function has no effect.

Syntax:

void std::string::reserve(size_t new_cap);

Example:

#include <iostream>
#include <string>

int main()
{
    std::string str;
    std::cout << "Before reserve(), capacity = "
              << str.capacity() << std::endl;
    str.reserve(100); // Reserve memory for 100 characters
    std::cout << "After reserve(), capacity = "
              << str.capacity() << std::endl;
    return 0;
}

Before reserve(), capacity = 0
After reserve(), capacity = 100

5. std::string::shrink_to_fit

• std::string::shrink_to_fit() reduces the capacity of the string object to fit its actual length, potentially saving memory. 
• The function has no effect if the current capacity is already equal to or less than the actual length of the string.
• The function can be used to release unused memory of a string object that was previously expanded with std::string::reserve(). 
• Calling std::string::shrink_to_fit() can reduce the capacity to fit the actual string, potentially saving memory.

Syntax:

void std::string::shrink_to_fit();

Example:

#include <iostream>
#include <string>

int main()
{
    std::string str = "Hello, World!";
    std::cout << "Before shrink_to_fit(), capacity = "
              << str.capacity() << std::endl;
    str.reserve(100); // Reserve memory for 100 characters
    std::cout << "After reserve(), capacity = "
              << str.capacity() << std::endl;
    str.shrink_to_fit(); // Shrink the capacity to fit the
                         // actual length
    std::cout << "After shrink_to_fit(), capacity = "
              << str.capacity() << std::endl;
    return 0;
}

Before shrink_to_fit(), capacity = 13
After reserve(), capacity = 100
After shrink_to_fit(), capacity = 13