Functions, take 1

A function in C++ is a code block which is executed only when the function is called. A function takes arguments, performs some action, and usually returns some result.

Here’s a generic example:

return_type function_name(type_1 argument_1, ..., type_N argument_N)
{
    // The function code block ("function body")

    // If the return_type is not "void", the function must 
    // end with a "return" statement.
}

Here’s a silly, specific example:

double add(double a, double b)
{
    return a + b; 
}

And here’s an example function with a void return type, meaning that the function simply performs some action and does not return anything back to the calling program:

void say_hello(std::string name)
{
    std::cout << "Hi there, " << name << "!" << std::endl;
}

Note that for such void functions, the function does not end with a return statement.

Declaration vs definition

In C++ there is a difference between a function declaration and a function definition:

• Function declaration: A single code statement that just specifies the function’s name, return type and arguments, e.g

  double add(double a, double b);
  

• Function definition: A complete definition of the function, e.g.

  double add(double a, double b)
  {
      return a + b; 
  }
  

Separation of declaration and definition

If the function definition appears before the point in the code where it is used (e.g. in the main function), no separate declaration line is needed:

#include <iostream>

// The function definition
double add(double x, double y)
{
    return a + y;
}

// Main function
int main() 
{
    double x = 1.2;
    double y = 3.4;

    double sum = add(x, y);

    std::cout << "The sum is " << sum << std::endl;

    return 0;
}

However, if the function definition is placed after the point where it is first used, a function declaration line must be added first:

#include <iostream>

// Function declaration
double add(double x, double y); 


// Main function
int main() 
{
    double x = 1.2;
    double y = 3.4;

    double sum = add(x, y);

    std::cout << "The sum is " << sum << std::endl;

    return 0;
}


// The function definition
double add(double x, double y)
{
    return a + y;
}

When a function is declared somewhere before the full function definition is provided, like in the example above, we refer to the declaration as a forward declaration.

In larger code projects, function declarations are typically placed inside header files (.hpp) that the main program can import through #include statements, while the corresponding function definitions live in source files (.cpp), completely separate from the .cpp file with the main program. In the next sections we will return to this approach for organising the code.