The **basic_string** is the generalization of *class string* for any character type. The string is an instantiation of the basic_string class that represents sequences of characters. 

The **basic_string class** provides a list of methods that are used for various operations. One of them is `std::basic_string::copy`. **`std::basic_string::copy`** copies a sequence of characters from a given string and returns the copied characters as an array.

# Parameters
As seen in the diagram, the `std::basic_string::copy` method accepts three parameters.

1. `buffer`: represents a pointer to an array of characters that contains the copied characters from the given string.
2. `len`: an integer that specifies the number of characters to be copied.
3. `pos`: an integer that specifies the position of the first character to be copied.

Given that we have a string named `str`, the syntax of `std::basic_string::copy` is:

> str.*copy(char\* buffer, len, pos);*

# Value returned
`std::basic_string::copy` returns the characters that are stored into the array and pointed to by the buffer (pointer). 

> Note: The length of the array pointed to by the buffer should be long enough to store the copied characters; otherwise, some weird output may appear. Also, the value given to the `pos` parameter must be less than or equal to the length of the original string; otherwise, it will give the `std::out_of_range` exception. 


# Code
Here, we have an array named `buffer` that contains the copied characters from the given string. As you can see, the `buffer`'s size is comparatively more than the number of characters that we want to copy. 

Upon executing this code, the characters from position `8` to position `18(10 + 8)` will be copied into a `buffer` and printed on the terminal.

#include <iostream>
#include <string>

int main ()
{
  char buffer[20];
  std::string str ("What is std::basic_string::copy in C/C++?");
  str.copy(buffer, 10, 8);
  std::cout << "Output: " << buffer;
  return 0;
}

Now, we will set the size of the array to less than the number of characters that we will copy. Any guess what the output will be?

#include <iostream>
#include <string>

int main ()
{
  char buffer[10];
  std::string str ("What is std::basic_string::copy in C/C++?");
  str.copy(buffer, 14, 8);
  std::cout << "Output: " << buffer;
  return 0;
}

Finally, we will set the `pos` parameter to greater than the length of the string. This time, I think we can easily guess the output.

#include <iostream>
#include <string>

int main ()
{
  char buffer[20];
  std::string str ("What is std::basic_string::copy in C/C++?");
  str.copy(buffer, 10, 42);
  std::cout << "Output: " << buffer;
  return 0;
}

That's all for `std::basic_string::copy`. I hope this shot has helped you understand this method.

> This method doesn't assign a null character at the end of the string on its own, so if you need one, you have to append the null character yourself.

You can connect with me on [Twitter](https://twitter.com/shubhHAM_) for any discussions.

Thanks for reading.

Adios!

What is std::basic_string::copy in C/C++?

std::basic_string::copy copies characters from a string to an array, given specified length and start position parameters. It doesn't append a null character.