Functions

What is a functions?

A function is the most basic form of abstraction in programming. They allow software to be broken down into more simple pieces of code and compose and reuse them as we please.

Much like functions in mathematics, functions in C++ (an every programming language) take in some input parameters (aka arguments or points) and return a single output value. This creates a transformation or mapping between input values and types to output values and types.

Function Syntax

Functions in C++ consist of a declaration and definition. A declaration is the functions signature which consists of the functions name, its points and its return type. The definition is a code block with at least on return expression.

T f(A1 a1, A2 a2)
{ 
    /* code */ 
    
    return ...;
}

• T is the return type
• A1 is the type of argument one stored in parameter a1
• A2 is the type of argument one stored in parameter a2
• return ...; is the return expression

Functions have to be invoked to be used. This involves using the invocation operator () on the functions name. You can pass literals and objects to functions. You can also initialise and assign variables from the return of a function.

#include <iostream>

int add(int x, int y)
{ return x + y; }

auto main () -> int
{
    auto a {4};
    auto b {7};
    auto c {-3};

    int d = add(5, 7);

    std::cout << add(a, b) << std::endl;
    std::cout << add(a, c) << std::endl;
    std::cout << add(a, d) << std::endl;
    std::cout << add(b, c) << std::endl;
    std::cout << add(b, d) << std::endl;
    std::cout << add(c, d) << std::endl;

    return 0;
}

Example

Functions allow us to abstract any common pattern into reusable code.

#include <iostream>

int sum(int s, int f)
{
    auto acc {0};
    for (auto i {s}; i < f; ++i)
        acc += i;
    return acc;
}

auto main () -> int
{

    std::cout << sum(0, 5) << std::endl;
    std::cout << sum(-3, 8) << std::endl;
    std::cout << sum(-11, -5) << std::endl;
    std::cout << sum(4, 19) << std::endl;

    return 0;
}

Example

Void Functions

Functions can also return nothing. This is often the case when functions have side effects. Side effects are operations that occur outside the input and outputs domains of a function. Printing with std::cout (or rather the underlying function that `std::cout calls) is an example of a function with side effects. It takes the string literal as input and returns, nothing but the effect of the text printing still occurs.

In C++, the lack of a return type is denoted by the void literal.

#include <iostream>

void println(auto s)
{ std::cout << s << std::endl; }

auto main () -> int
{
    println("Hello World!");

    return 0;
}

Example