C++11: Standard ref for action of `auto` on const and reference types

Question

Suppose I have a type T:

typedef ... T;

and then I have these functions:

T f11();
T& f12();
T&& f13();
const T f21();
const T& f22();
const T&& f23();

and then call them like this:

auto x11 = f11();
auto x12 = f12();
auto x13 = f13();
auto x21 = f21();
auto x22 = f22();
auto x23 = f23();

From which sections/clauses of the C++11 standard can it be deduced the equivalent non-auto declarations of x11..x23?

Now hold on. `typedef ... T;` makes any normal compiler spits out compilation errors — BЈовић, Sep 03 '12 at 07:01
@BЈовић: `...` in this context is a placeholder for any valid type. (not helped by the fact that `...` is also valid C++ in some places) — Andrew Tomazos, Sep 03 '12 at 08:16

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

5

It is in §7.1.6.4 auto specifier. In your examples of function return types, the rules of template argument deduction apply.

Paraquoting the relevant example from the standard:

const auto &i = expr;

The type of i is the deduced type of the parameter X in the call f(expr) of the following invented function template:

template <class AUTO> void f(const AUTO& X);

So in your examples, the types of all your variables x11 to x23 are deduced as T.

edited Jun 20 '20 at 09:12

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answered Sep 03 '12 at 05:36

juanchopanza

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That is if `template g(T t);` than `g(f())` and `auto t = f()` would result in the same type of t in both cases? – Andrew Tomazos Sep 03 '12 at 05:41
@AndrewTomazos-Fathomling the example in the standard is `const auto&i = expr` and `template void f(const U& u);`. Here, the type of `i` is the deduced type of `u` in `f(expr)`. – juanchopanza Sep 03 '12 at 05:52
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In particular, this means that the type of each deduced variable will be `T`. – Luc Danton Sep 03 '12 at 06:06
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Maybe you should have written `template ` to make the point... :-) – Kerrek SB Sep 03 '12 at 06:16
@KerrekSB I have adopted your suggestion. I wasn't to keen on modifying a quote from the standard until I discovered "paraquote". – juanchopanza Sep 03 '12 at 12:04

C++11: Standard ref for action of `auto` on const and reference types

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