include

int main() { std::cout << "Hello, world!" << std::endl; return 0; }

Explanation: 1. The first line, "#include ", includes the input/output stream library in the C++ program. This library provides functionality for handling input and output operations.

1. The "int main()" line declares the main function of the program. In C++, every program must have a main function, and program execution starts from here.

2. The opening curly brace "{" denotes the start of the main function.

3. The line "std::cout << "Hello, world!" << std::endl;" is responsible for printing the string "Hello, world!" to the console.

4. "std::cout" is an object of the ostream class, which is used for outputting data. It is followed by the "<<" operator, which is used to insert data into the ostream object.

5. The string "Hello, world!" is enclosed in double quotes and is the data that is being inserted into the ostream object.

6. "std::endl" is used to insert a newline character into the output stream, effectively moving the cursor to the next line.

7. The line ends with a semicolon ";" which marks the end of the statement.

8. The closing curly brace "}" denotes the end of the main function.

9. The "return 0;" line is used to exit the main function and return a value of 0 to the operating system. This value indicates that the program executed successfully.

10. The closing curly brace "}" denotes the end of the program.