Functions (Basics) Functions (Basics) Functions

Inputs & Outputs In/Out

First Example Example

Function that computes mean of 2 numbers

#include <iostream>
double mean (double a, double b) {
  return (a + b) / 2;
}

int main () {
  std::cout << mean(2, 6) <<'\n';  // prints 4
}

encapsulation of implementation details
easier reasoning about correctness and testing by breaking down problems into separate functions
avoids repeating code for common tasks

// "call" at "call site"
auto result = name(argument1, argument2);

// "call" at "call site"
auto result = name(argument1, argument2);

Return Types

either one value: `int`, `double`, …

double square (double x) {
  return (x * x);
}
int max (int x, int y) {
  if (x > y) return x; else return y;
}

or nothing: `void`

void print_squares (int n) {
  for (int i = 1; i <= n; ++i)
    cout << square(i) << '\n';
}

Full Return Type Deduction C++14 (deduction = compiler determines type automatically)

auto foo (int i, double d) {
  …
  return i;
}
//  OK: return type: int

auto foo (int i, double d) {
  return i;  //  int
  …
  return d;  //  double
} 
//  ERROR: Inconsistent return types!

Parameters

none: f()
one or many: g(int a, double b, int c, …)
parameter names have to be unique within list

`const` Parameters `const`

int foo (int a, int const b) {
  a += 5;   // 
  b += 10;  //  COMPILER ERROR: can't modify const parameter
  return (a + b);
}
// calling foo:
foo(2,9);  // const has no effect here

Any 2^nd argument passed to foo will be copied into the local variable b ⇒ the fact that b is const has no effect outside of foo.

If you don't need or must not change values of parameters inside the function then make them const!

Defaulted Parameters Defaults

double f (double a, double b = 1.5) {
  return (a * b);
}
int main () {
  cout <<  f(2);     // 1 argument  → 3.0
  cout <<  f(2, 3);  // 2 arguments → 6.0
}
void foo (int i = 0);  
void foo (int n, double x = 2.5);  
void foo (int a, int b = 1, float c = 3.5f);  
void foo (int a, int b = 1, int c );

Each parameter after first default must have default value, too!

Overloading

functions with the same name but different parameter lists
cannot overload on return type alone

same name, different parameter lists

int abs (int i) {
  return ((i < 0) ? -i : i);
}

double abs (double d) {
  return ((d < 0.0) ? -d : d);
}
int a = -5;
double b = -2.23;
auto x = abs(a); // int abs(int)
auto y = abs(b); // double abs(double)

same name, same parameter lists

int foo (int i) { 
  return (2 * i);
}

double foo (int i) { 
  return (2.5 * i);
}

DOES NOT COMPILE!

Implementation

Recursion

= function calling itself

needs a break condition
looks more elegant than loops but in many cases slower
recursion depth is limited (by stack size)

int factorial (int n) {
    // break condition:
    if (n < 2) return 1;  
    // recursive call: n! = n * (n-1)!
    return (n * factorial(n - 1));  
}

Declaration vs. Definition Declaration

can only call functions that are already known (from before/above)
only one definition allowed per source file (translation unit)
ok to have any number of declarations = announcing the existence of a function by specifying its signature

Example: Broken! (click!)

#include <iostream>
//  COMPILER ERROR: - 'odd'/'even' not known before 'main'!
//  COMPILER ERROR: - 'odd' not known before 'even'!

int main () {
  std::cout << "enter an integer: ";  int i = 0;
  std::cin >> i;
  if (odd(i))  std::cout << "is odd\n";
  if (even(i)) std::cout << "is even\n";
}

bool even (int n) {
  return !odd(n);
}

bool odd (int n) {
  return (n % 2);
}

Working (click!)

#include <iostream>
bool even (int);  // declaration
bool odd (int);   // declaration

int main () {     // definition
  std::cout << "enter an integer: ";  int i = 0; 
  std::cin >> i;
  if (odd(i))  std::cout << "is odd\n";   // OK, already declared
  if (even(i)) std::cout << "is even\n";  // OK, already declared
}

bool even (int n) { // definition
  return !odd(n);  // OK, already declared
}

bool odd (int n) {  // definition
  return (n % 2);
}

Design

Interfaces should be easy to use correctly and hard to use incorrectly.
— Scott Meyers

Contracts

When designing a function, think about:

Preconditions: What do you expect/demand from input values?
Postconditions: What guarantees should you give regarding output values?
Invariants: What do callers/users of your function expect to not change?
Purpose: Has your function a clearly defined purpose?
Name: Does the function's name reflect its purpose?
Parameters: Can a caller/user easily confuse their meaning?

overview of function contract terminology: preconditions, postconditions, invariants, side effects

Precondition Checks

Wide Contract Functions perform precondition checks, i.e., check input parameter values (or program state) for validity

Narrow Contract Functions do not perform precondition checks, i.e., the caller has to make sure that input arguments (and program state) are valid

Attribute `[[nodiscard]]` `[[nodiscard]]` `[[nodiscard]]` C++17

encourages compilers to issue warnings if function return values are discarded

[[nodiscard]] bool prime (int i) { … }
// return value(s) used:
bool const yes = prime(47);
if (prime(47)) { … }
// return value discarded/ignored:
prime(47);  //  COMPILER WARNING

Example from the standard library:

std::vector's empty() function is declared with [[nodiscard]] as of C++20, because it can be confused with clear():

std::vector<int> v;
// …
if (v.empty()) { … }  // OK
v.empty();  // C++20:  COMPILER WARNING
// oops … did someone meant to clear it?

Declare your function return values `[[nodiscard]]`

if calling it without using the return value makes no sense in any situation
if users could be confused about its purpose, if the return value is ignored

No-Throw Guarantee: `noexcept` `noexcept` `noexcept` C++11

C++ has a mechanism for reporting errors using exceptions like many/most other programming languages. Don't worry, if you don't know what exceptions are, they will be explained in a later chapter.

The noexcept keyword specifies that a function promises to never throw exceptions / let exceptions escape:

void foo () noexcept { … }

If an exception escapes from a noexcept function anyway, the program will be aborted.

Some Mathematical Functions Mathematical Functions Math

`#include <cmath>`
`double sqrt (double x)`	√x	square root
`double pow (double a, double b)`	a^b	power
`double abs (double x)`	\|x\|	absolute value
`double sin (double x)`	sin(x)	sine
`double cos (double x)`	cos(x)	cosine
`double exp (double x)`	e^x	exponential
`double log (double x)`	log(x)	logarithm
`double floor (double x)`	⌊x⌋	next smaller integer
`double ceil (double x)`	⌈x⌉	next larger integer
`double fmod (double x, double y)`	remainder of x/y

Type Basics

Beginner's Guide / First Steps

Memory

More About Functions (more advanced material)(skips ahead to more advanced material):

Last updated: 2021-05-12

First Steps

Input & Output

Custom Types – Part 1

Diagnostics

Standard Library – Part 1

Function Objects

Standard Library – Part 2

Code Organization

Custom Types – Part 2

Generic Programming

Memory Management

Software Design Basics