Beginner's Guide

# Functions (Basics)Functions (Basics)Functions

## Inputs & OutputsIn/Out

### First Example

``````#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

###### 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 2nd 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 ParametersDefaults

``````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!

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

## 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. DefinitionDeclaration

• 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 of 'main'
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 of 'even'
return !odd(n);  // OK, already declared
}
bool odd (int n) {  // definition of 'odd'
return (n % 2);
}``````

## Design

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

### Contracts

• 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?

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

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

`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?``````
• 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 FunctionsMathematical FunctionsMath

 `#include ` `double sqrt (double x)` √x square root `double pow (double a, double b)` ab 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)` ex 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