Patch 1

book/hello.Rmd

@@ -25,12 +25,8 @@ Every C++ executable (as opposed to library) must have a `main()` function that
 returns `int`. Returning `0` signifies that the program terminates without errors.
 The final `return 0;` statement can be omitted in the `main()` function.
 
-`#include` is a preprocessor. We'll meet more preprocessors in the future, for now
- just accept that they are "naive macros" that are "expanded" before the actual
- compilation.
-Here `#include` copies the content of file called `iostream`, which has tens of
-thousands lines, and pastes it here. Yes, it literally does so, and you can check
-this by running `g++ -E main.c`, which "expands" all preprocessor statements.
+`#include` is a preprocessor directive. We'll learn more about the preprocessor in later chapters, for now just accept that preprocessor directives are "naive macros" that are "expanded" before the actual compilation.
+Here `#include` copies the content of file called `iostream`, which has tens of thousands lines, and pastes it here. Yes, it literally does so, and you can check this by running `g++ -E main.c`, which "expands" all preprocessor statements.
 
 `iostream` contains definitions of functions and objects such as `std::cout` and
 `std::endl`, which are used for IO manipulations. `cout` stands for "character
@@ -131,7 +127,7 @@ int a{5};
 knitr::include_graphics("img/equal.png")
 ```
 
-If you try `int a{5.5};` with this syntax, the compiler will give an error and abort (Figure \@ref(fig:braces) ). In addition, you can't separate the two parts:
+If you try `int a{5.5};` with this syntax, the compiler will give an error and abort (Figure \@ref(fig:braces) ). In addition, you can't separate the two parts, thus enforcing that variables are not left uninitialized:
 
 ```c++
 // not allowed
@@ -145,6 +141,30 @@ knitr::include_graphics("img/brace.png")
 
 Of course, the uniform initialization syntax isn't invented just to prevent implicit conversion. As you'll see later, it can become handy when initializing complicated, non-primitive data types.
 
+###`auto` vs `let`
+
+Another key difference with declaring / initializing variables is in how types are inferred. In C++, the type of a variable can be inferred by replacing the type name with `auto`:
+
+```c++
+//These lines are equivalent
+auto a = 5;
+int a = 5;
+```
+
+Unlike Rust's `let` keyword, a variable declared with `auto` MUST be initialized in the same statement - that is, you cannot declare the variable with `auto`, then have the compiler infer the type from an initialization in another statement. Example:
+
+```rust
+// Rust - valid, type of a will be inferred as f32
+let a;
+a = 10. as f64;
+```
+
+```c++
+// C++ - invalid, variable must be initialized and declared in the same statement
+auto a;
+a = 10.;
+```
+
 ### Mutability
 
 Variables are mutable by default. If you want to create an immutable variable, use the `const` keyword.
@@ -752,4 +772,4 @@ int& rj = j;
 ri = rj;
 // What are the values of i, j, ri and rj now?
 // Hint: a reference cannot be re-assigned. What's the last line doing?
-```
+```