Preview of Functions

We have already been using C++ functions, and soon we’ll learn how to write our own.

For example, std::sin() from <cmath> is a function. Any function in C++ will:

• Take arguments passed in through the ().

  For example, we do std::sin(x), and here x is an argument.

  Some functions will take multiple arguments (separated by ,) and some may not take any, as with epsilon() in the line:

  std::numeric_limits<double>::epsilon()
  

• Return a value.

  For both std::sin(x) and the epsilon(), these functions returned a double, which we capture via assignment, e.g,:

  double x{};
  double y = std::sin(x);
  

The one function that we have been writing so far is main(), which has a return type of int.

Function prototype

We call the combination of the return type, function name, and arguments, without the function body the prototype. This is the declaration for the function.

A prototype has the form:

return-type  name (argument1, argument2, ...);

The actual implementation of the function can be provided elsewhere (again, more on this later…).

For our sin(x) function, we would find the prototype / declaration in the cmath header. On my machine, this is in /usr/include/c++/15/

It could take the form:

double sin(double x);