Differences between C# "var" and C++ "auto"

Question

I'm learning C++ now because I need to write some low level programs.

When I learned about "auto" keyword, it reminds me "var" keyword, from C#.

So, what are differences of C# "var" and C++ "auto"?

Answer 1

In C# var keyword works only locally inside function:

var i = 10; // implicitly typed  

In C++ auto keyword can deduce type not only in variables, but also in functions and templates:

auto i = 10;  auto foo() { //deduced to be int     return 5; }  template<typename T, typename U> auto add(T t, U u) {     return t + u; } 

From performance point of view, auto keyword in C++ does not affect runtime performance. And var keyword does not affect runtime performance as well.

Another difference can be in intellisense support in IDE. Var keyword in C# can be easily deduced and you will see the type with mouse over. With auto keyword in C++ it might be more complicated, it depends on IDE.

Answer 2

To put things simply, auto is a much more complicated beast than var.

First, auto may only be part of the deduced type; for example:

std::vector<X> xs; // Fill xs for (auto x : xs) x.modify(); // modifies the local copy of object contained in xs for (auto& x : xs) x.modify(); // modifies the object contained in xs for (auto const& x : xs) x.modify(); // Error: x is const ref 

Second, auto may be used to declare multiple objects at once:

int f(); int* g(); auto i = f(), *pi = g(); 

Third, auto is used as part of the trailing return type syntax in function declarations:

template <class T, class U> auto add(T t, U u) -> decltype(t + u); 

It may also be used for type deduction in function definitions:

template <class T, class U> auto add(T t, U u) { return t + u; } 

Fourth, in the future it may start to be used to declare function templates:

void f(auto (auto::*mf)(auto)); // Same as: template<typename T, typename U, typename V> void f(T (U::*mf)(V));