> For the complete documentation index, see [llms.txt](https://learncpp.benntan.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://learncpp.benntan.com/cpp-fundamentals/basic-syntax-and-i-o.md). # Basic Syntax & I/O Let's kick things off by familiarising ourselves with the basic structure of a C++ program. ### Anatomy of a C++ Program Recall the source code you previously saw: {% code title="main.cpp" %} ```cpp #include int main() { std::cout << "Hello, world!"; return 0; } ``` {% endcode %} Breaking it down:

Code	What it does
`#include <iostream>`	Brings in the standard I/O library so that we can use things like `std::cout`
`int main() {`	The entry point of every C++ program. `int` indicates the return type of the function `main` is the function name.
`std::cout << "Hel...`	Prints the text to the console. 👉 You can remember `<<` as pushing the text into the output stream (i.e `std::cout` in this case)
`return 0;`	Recall that `int` is the return type of the `main` function. Hence, at the end of our `main` function, we should^[1] return an int. You can return anything, really. But usually returning 0 indicates our program has successfully executed.

{% hint style="info" %} Try augmenting the `Hello, world!` string to something else like your name! ![](/files/0pHm0U6QhZE9yFEu9ug8) {% endhint %} _{\[1] Your code will still compile even if you don't return anything from the main function. This is because C++ implicilty returns 0 at the end of} _`main()`_{. For more info, refer} [_here](https://stackoverflow.com/questions/19293642/why-does-the-main-function-work-with-no-return-value)_. ### Newlines If you try printing multiple times, you will notice that the output gets concatenated on the same line. You can fix this by adding a newline using `std::endl` .

Code Output

Code	Output
`#include <iostream> int main() { std::cout << "benn"; std::cout << "tan"; std::cout << "jia"; return 0; }`
`#include \ int main() { std::cout << "benn" << std::endl; std::cout << "tan" << std::endl; std::cout << "jia"; return 0; }`

#include <iostream>

int main()
{
std::cout << "benn";
std::cout << "tan";
std::cout << "jia";
return 0;
}

#include \

int main()
{
std::cout << "benn" << std::endl;
std::cout << "tan" << std::endl;
std::cout << "jia";
return 0;
}

### Input It's not quite fun if we hardcode everything into program. Let's augment our program to accept user input externally. To read input from the user, we can use `std::cin >>` {% code title="main.cpp" %} ```cpp #include int main() { int x; std::cin >> x; std::cout << "You entered: " << x << std::endl; } ``` {% endcode %} {% hint style="info" %} Note that the direction of the arrows (`>>`) for `std::cin` is now reversed compared to `<<`. 👉 You can remember as **data flowing into `x`**. {% endhint %} ### Namespaces If you find prefixing `std::` before the function calls cumbersome, you can omit it with `using namespace std` {% code title="main.cpp" %} ```cpp #include using namespace std; int main() { int x; cin >> x; cout << "You entered: " << x << endl; } ``` {% endcode %} However, this is generally discouraged in larger projects as it does lead to naming conflicts. 👉 It's usually best practice to be explicit and use `std::` ### Exercise Build a program that takes in and prints the result of the summation of two numbers. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://learncpp.benntan.com/cpp-fundamentals/basic-syntax-and-i-o.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.