Runnable code block

Add run tag to the language declaration of the code block to enable running:

```cpp, run
#include <iostream>
int main() {
    std::cout << "good" << std::endl;
    std::cout << "hello from cpp!" << std::endl;
    return 0;
}
```

rendered as:

#include <iostream>
int main() {
    std::cout << "good" << std::endl;
    std::cout << "hello from cpp!" << std::endl;
    return 0;
}

Editable code

Add editable to enable editing:

```cpp, editable, run
#include <iostream>
int main() {
    std::cout << "hello from cpp!" << std::endl;
    return 0;
}
```

rendered as:

#include <iostream>
int main() {
    std::cout << "hello from cpp!" << std::endl;
    return 0;
}

or you prefer rust:

fn main() {
    let number = 5;
    print!("{}", number);
}

Hide lines in code block

Sometimes we just want the reader to focus on the important part of the code.

Note:

Editable code blocks can not use this feature.

Rust

In rust, we can use # at the beginning of the line to automatically ignore that line:

```rust
# #![allow(unused)]
# fn main() {
println!("Hello, World!");
# }
```

rendered as:

#![allow(unused)]
fn main() {
println!("Hello, World!");
}

C++

In C++, we use ~ (note that there is a space after ~) at the beginning of the line to automatically ignore that line:

```cpp
~ #include <iostream>
~ int main() {
std::cout << "hello from cpp with hidden lines!" << std::endl;
~ return 0;
~ }
```

#include <iostream>
int main() {
std::cout << "hello from cpp with hidden lines!" << std::endl;
return 0;
}

Edition

Edition in the language identifier is used to specify which C++ standard you want to use for the compiler (default is C++11).

You can use:

```cpp, run, edition17
#include <iostream>
#include <string>
#include <optional>
int main() {
    std::optional<std::string> maybeName;
    maybeName = "Tom";
    if (maybeName) {
        std::cout << "Optional has value: " << *maybeName << "\n";
    }
    return 0;
}
```

to specify the standard as C++17, and it will be rendered as:

#include <iostream>
#include <string>
#include <optional>
int main() {
    std::optional<std::string> maybeName;
    maybeName = "Tom";
    if (maybeName) {
        std::cout << "Optional has value: " << *maybeName << "\n";
    }
    return 0;
}

and if you use C++11, it is expected that you see some compile error:

#include <iostream>
#include <string>
#include <optional>
int main() {
    std::optional<std::string> maybeName;
    maybeName = "Tom";
    if (maybeName) {
        std::cout << "Optional has value: " << *maybeName << "\n";
    }
    return 0;
}

Exception

There are some exceptions you might run into:

Timeout

The time limit is set to 2 seconds of each process.

So this is ok:

#include <unistd.h>
int main() {
    sleep(1);
    return 0;
}

while this is not:

#include <unistd.h>
int main() {
    sleep(2);
    return 0;
}

Out of memory

The heap memory limit is set to 20 MiB.

So this is not ok:

#include <vector>
int main() {
    // int is 4 bytes on the server
    std::vector<int> vec = std::vector<int>(5 * 1024 * 1024);
    return 0;
}

Actually, std::vector<int>(4 * 1024 * 1024); also not working, though I don't know why. I use the setrlimit in the <sys/resource.h> to set the limit, maybe there is some soft limit & hard limit thing?

Privilege violation

There is a limit to restrict you from directly manipulating the OS.

This is not ok:

#include <cstdlib>
int main() {
    system("ls");
    return 0;
}

Net Nook