To keep function declarations separate from their definitions in C++, Ethan can use header files (.h) for declarations and source files (.cpp) for definitions. This practice enhances code organization, reusability, and maintainability in large projects.

When a floating-point number is assigned to an integer type in C++, the value will be truncated. This means that the fractional part of the floating-point number is discarded, and only the whole number part is stored in the integer variable. It's essential to be aware of this behavior to prevent unexpected results in your code.

The for loop is best suited for scenarios where the number of iterations is known beforehand. It's a compact loop structure that allows you to initialize a variable, define a condition, and specify an increment or decrement in a single line. This makes it ideal for loops with a fixed number of iterations, such as iterating through arrays or performing a set number of calculations.

If two functions in a class have the same name but differ in the number or type of parameters, it's referred to as "Function Overloading." This allows you to create multiple functions with the same name, providing flexibility and clarity when using them in different contexts.

For a function whose return type is deduced at the time of its invocation, the keyword 'auto' is used. This is part of the C++11 feature where the compiler determines the return type based on the function's implementation. It allows for more flexible and concise code.

In C++, to store the value of pi up to several decimal places, you would typically use the double data type. The double data type provides greater precision compared to float or int, making it suitable for representing the value of pi with more decimal places.

In C++, the conditional operator "?:" is commonly known as the ternary operator. It is a shorthand way to write conditional expressions and is often used to assign values based on a condition. The operator takes three operands: a condition, a value if the condition is true, and a value if the condition is false.

To ensure portability, you can use fixed-size integer data types like int32_t from the header. These types guarantee a consistent size across platforms, making your code less dependent on the underlying system's architecture. This is crucial when dealing with binary data or network protocols.

Function overloading can be used in conjunction with functions that have default arguments. In C++, you can have multiple functions with the same name but different parameter lists (overloads), and some of these overloads can have default arguments. The compiler selects the appropriate function based on the arguments provided.

Emily should use the 'double' data type for better precision in calculations involving decimal numbers, such as the area of a circle. 'Double' provides a higher level of precision compared to 'float' and is suitable for scientific and engineering calculations.