How is a function defined in C++?

In C++, a function is defined as a block of code that performs a specific task and can be reused. This characteristic of reusability is one of the fundamental principles of programming. Functions allow programmers to write a piece of code once and then call it multiple times from different parts of a program, which enhances code organization, readability, and maintainability.

When a function is created, it typically consists of a function header that includes the return type, the function name, and parameters (if any). The body of the function contains the specific statements that define what the function does. This modular approach enables developers to break down complex tasks into smaller, more manageable pieces, making it easier to understand and debug code.

The other choices do not accurately describe the nature of functions in C++. For instance, calling a function reusable is a critical aspect of its definition, while defining it purely as a block of code that cannot be reused would contradict this principle. Similarly, functions are not data types or variables that hold values; they are distinct constructs that perform operations and can return values to the part of the program that called them.