C auto keyword Why is it magic

Question

Auto has always been a strange storage duration specifier that served no purpose in all of the materials I used to learn C++.

However, I recently came across code that used it as a type name by itself.

I tried it out of curiosity, and it takes on the type of whatever I assign to it!

STL iterators and, well, anything that uses templates is suddenly ten times easier to write.

It's as if I'm programming in a 'fun' language like Python.

Where have I been looking for this keyword my entire life?

Will you break my heart by telling me it's only available in Visual Studio or that it's not portable?

Answer 1

Auto was a keyword that C++ "inherited" from C and had been around for a long time but was almost never used because there were only two possibilities: it wasn't allowed or it was assumed by default.

C++11 introduced the usage of auto to denote an inferred type.

Similarly to how template type deduction works for function templates, auto x = initializer deduces the type of x from the type of initializer. 

Consider the following function template:

template<class T>
int whatever(T t) { 
    // point A
};

T has been allocated a type at point A based on the value supplied for whatever's argument. 

When you execute auto x = initializer;, the type for x is deduced from the type of initializer that was used to initialize it.

This implies that for every compiler that attempted to implement C++98/03, much of the type deduction mechanisms needed to construct auto were already present and utilised for templates. 

As a result, adding support for auto appears to have been rather simple for virtually all compiler teams—it was implemented rapidly, and there appear to have been few errors associated with it.