Learn Rust With Me

For C++ Programmers

This is a quick Rust guide for C++ programmers.

Program Entry Point

Like C/C++ the entry point for a program is the main function:

fn main() {
    println!("hello");
}

Variable Declaration

Variables are declared using the let keyword, analogous to C++'s auto keyword. Variable types can be explicitly stated or implicitly deduced from values, expressions, or functions.

Explicit Type

let i : i32 = 1;   // i32
let u : u32 = 1;   // u32
let x : f32 = 1.0; // f32
let y : f64 = 1.0; // f64

// -- or --

let i = 1i32; // i32
let u = 1u32; // u32
let x = 1f32; // f32
let y = 1f64; // f64

Deduced Type From Value

let i = 1;       // i32
let x = 1.0;     // f64
let s = "hello"; // &str

Deduced Type From Expression

let x = 1.0 as f32; // f32
let y = x + 2.0;    // f32

Deduced Type from Function

Variable types can be deduced from values, expressions, and functions:

fn add(a i32, b: 32) -> i32 {a + b}

let i = add(1, 2); // i32

Type Strictness

Rust has very strict type requirements and there is not implicit conversion like C/C++. Mixing types will generally result in a compiler error in Rust.

let x = 1.0 + 2;        // ERROR: cannot add an integer to a float
let y = 1.0 + 2 as f32; // OK
let z = 1.0 + 2f32;     // OK

Return Values

Return types are specified using the -> operator at the end of a function signature:

fn add(a: i32, b: 32) -> i32 { a+b }

The use of the return keyword can be optional. The last statement in the function without a ; is implicitly the return value.

// Implicit return, no ; here
fn add(a: i32, b: i32) -> i32 {
    a + b
}

// Explicit return, must have ; when using return keyword
fn add(a: i32, b: i32) -> i32 {
    return a + b;
}

Conditionals

if and while statements in Rust require curly braces but parentheses are optional:

if cond {}
if cond1 && cond2 {}
if cond1 {}
if (var == true) {}
if (var1 == true) || (var2 == true) {}
if ((var1 == true) || (var2 == true)) {}

while cond {}
while cond1 && cond2 {}
while cond1 {}
while (var == true) {}
while (var1 == true) || (var2 == true) {}
while ((var1 == true) || (var2 == true)) {}

Ternary Operator

There is not a ternary operator, but the equivalent expression can be written using if/else:

let x = if cond { true } else { false }

Notice that there is not a ; in the branches of if/else.