The built-in function **`append()`** is defined in the `<string>` header. This function is used in _string concatenation_, similar to the `+` operator. 

It appends specified characters at the end of a `string`, extending its length. 

# 
## Return value

The function returns `*this`, i.e., the change is made directly to the `string` that calls this function.

This is true for all the different ways of using the function.

# 
## Syntax

The function has different syntaxes that we can use, as shown below.

# 
##   
## 1. String

---
```
string& append (const string& str);
```
---

The first syntax has only one argument:

1. `str`: the `string` to be added at the end of the `string` that calls this function.



#include <iostream>
using namespace std;

int main() {
  string str1 = "Learn C++ ";
  string str2 = "with Educative!";

  cout << "String without appending: " << str1 << endl;

  str1.append(str2);

  cout << "String after appending: " << str1;

  return 0;
}

> Similarly, a C-string can also be used in the `append()` function.

# 
##   
## 2. Substring

---
```
string& append (const string& str, size_t subpos, size_t sublen);
```
---

This syntax takes three arguments:

1. `str`: the `string` from which the characters will be appended to the `string` that calls this function.

2. `subpos`: the starting index of the substring to be appended.

3. `sublen`: the length of the substring to be appended.



#include <iostream>
using namespace std;

int main() {
  string str1 = "Hello from ";
  string str2 = "with Educative!";

  cout << "String without appending: " << str1 << endl;

  str1.append(str2, 5, 10);

  cout << "String after appending: " << str1;

  return 0;
}

> Remember that indexing starts from 0 in C++.

# 
##   
## 3. Fill

---
```
string& append (size_t n, char c);
```
---

This syntax takes two arguments:

1. `n`: the number of times `c` should be added.

2. `c`: the `char` that needs to be appended at the end of the `string`.

#include <iostream>
using namespace std;

int main() {
  string str1 = "Hello from Educative";

  cout << "String without appending: " << str1 << endl;
  str1.append(4, '!');

  cout << "String after appending: " << str1;

  return 0;
}

# 
##   
## 4. Buffer

---
```
string& append (const char* s, size_t n);
```
---

This syntax takes two arguments:

1. `s`: the c-string from which the characters will be appended.

2. `n`: the number of characters to be appended.

#include <iostream>
using namespace std;

int main() {
  string str1 = "Hello from ";

  cout << "String without appending: " << str1 << endl;
  str1.append("Educative!!!", 9);

  cout << "String after appending: " << str1;

  return 0;
}

What is std::string::append() in C++?

In C++, `std::string::append()` adds characters to a string, using different syntaxes for strings, substrings, characters, or buffers.