C++ Types Zoo

A comprehensive exploration of auto and decltype type deduction.

Compiler: GCC 11.4

C++ Standard: 201703

1. Base Variable Types

Variable	Declared Type
x	int
cx	const int
rx	int&
rcx	const int&
rrx	int&&
rrcx	const int&&
px	int*
pcx	const int*
cpx	int* const
cpcx	const int* const

2. decltype on Expressions

2.1 decltype on Variables (id-expressions)

When decltype is applied to an unparenthesized id-expression, it yields the declared type.

Expression	Type
decltype(x)	int
decltype(cx)	const int
decltype(rx)	int&
decltype(rcx)	const int&
decltype(rrx)	int&&
decltype(rrcx)	const int&&

2.2 decltype on Parenthesized Expressions

When decltype is applied to a parenthesized expression (expr), it considers the value category:

• lvalue → T&
• xvalue → T&&
• prvalue → T

Expression	Type	Why
decltype((x))	int&	x is lvalue
decltype((cx))	const int&	cx is lvalue
decltype((rx))	int&	rx is lvalue
decltype((rrx))	int&	rrx is lvalue!
decltype((std::move(x)))	int&&	xvalue
decltype((42))	int	prvalue
decltype((get_value()))	int	prvalue
decltype((get_lref()))	int&	lvalue
decltype((get_rref()))	int&&	xvalue

3. auto Type Deduction

3.1 Plain auto (strips references and top-level cv-qualifiers)

Declaration	Deduced Type
auto v = x	int
auto v = cx	int
auto v = rx	int
auto v = rcx	int
auto v = rrx	int
auto v = std::move(x)	int
auto v = get_value()	int
auto v = get_lref()	int
auto v = get_rref()	int

3.2 auto& (lvalue reference, preserves const)

Declaration	Deduced Type
auto& v = x	int&
auto& v = cx	const int&
auto& v = rx	int&
auto& v = rcx	const int&
auto& v = rrx	int&
auto& v = get_lref()	int&

Note: auto& v = std::move(x), auto& v = get_value(), auto& v = get_rref() would be errors (can't bind lvalue ref to rvalue).

3.3 const auto& (extends lifetime of temporaries)

Declaration	Deduced Type
const auto& v = x	const int&
const auto& v = cx	const int&
const auto& v = rx	const int&
const auto& v = std::move(x)	const int&
const auto& v = get_value()	const int&
const auto& v = 42	const int&

3.4 auto&& (forwarding/universal reference)

Declaration	Deduced Type	Why
auto&& v = x	int&	x is lvalue
auto&& v = cx	const int&	cx is lvalue
auto&& v = rx	int&	rx is lvalue
auto&& v = rcx	const int&	rcx is lvalue
auto&& v = rrx	int&	rrx is lvalue!
auto&& v = std::move(x)	int&&	xvalue
auto&& v = get_value()	int&&	prvalue
auto&& v = get_lref()	int&	lvalue
auto&& v = get_rref()	int&&	xvalue
auto&& v = 42	int&&	prvalue

4. decltype(auto)

Preserves the exact type including references and cv-qualifiers.

Declaration	Deduced Type	Why
decltype(auto) v = x	int	id-expression → declared type
decltype(auto) v = cx	const int	id-expression → declared type
decltype(auto) v = std::move(x)	int&&	expression returns int&&
decltype(auto) v = (x)	int&	(x) is lvalue → adds &
decltype(auto) v = (cx)	const int&	(cx) is lvalue → adds const&

Warning: decltype(auto) v = rrx; would be an error because rrx has declared type int&&, but rrx itself is an lvalue and can't bind to int&&.

5. Pointers with auto

Declaration	Deduced Type	Note
auto v = px	int*
auto v = pcx	const int*	pointee const preserved
auto v = cpx	int*	top-level const stripped
auto v = cpcx	const int*	top-level const stripped
auto* v = px	int*
auto* v = pcx	const int*	pointee const preserved
const auto* v = px	const int*	adds pointee const
auto* const v = px	int* const	pointer is const

6. Value Categories Reference

          expression
          /        \
      glvalue      rvalue
      /     \     /     \
   lvalue   xvalue   prvalue

Category	Has Identity	Can Move From	Examples
lvalue	Yes	No	x, *p, a[n], ++i
xvalue	Yes	Yes	std::move(x), a[n] where a is rvalue
prvalue	No	Yes	42, x + y, function returning by value

7. Key Insights

1. auto strips top-level const and references — it always creates a new copy
2. auto& preserves const but requires an lvalue
3. auto&& is a forwarding reference — binds to anything, preserves value category
4. decltype(var) returns declared type of the variable
5. decltype((var)) adds reference based on value category (lvalue → T&)
6. decltype(auto) uses decltype rules on the initializer expression
7. Named rvalue references are lvalues! — rrx in int&& rrx = ... is an lvalue
8. const auto& extends lifetime of temporaries
9. auto* preserves pointee const but auto strips top-level pointer const

8. Common Gotchas

Named rvalue references are lvalues

int&& rrx = std::move(x);  // rrx has type int&&
auto&& v = rrx;            // but rrx is an lvalue! v is int&

decltype with parentheses

int x = 42;
decltype(x)   a;  // int
decltype((x)) b = x;  // int& (dangerous: returns reference to local!)

auto strips const from the thing being copied

const int cx = 42;
auto v = cx;  // v is int, not const int

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