When using \'auto\' in C 11, how does it determine whether a variable is a reference or a value?

C 11 "auto" Type Deduction: Resolving Reference vs. Value

In C 11, the "auto" keyword provides a convenient way to automatically deduce the type of a variable. However, when using "auto," it's essential to understand the rules that determine whether it resolves to a value or a reference.

Type Deduction Rules for "auto":

The fundamental rule is that "auto" interprets the declaration of the variable itself, not the type it represents. Therefore, the following example clearly demonstrates that "auto" resolves to a value:

<code class="cpp">auto i = v.begin(); // Copy, as begin() returns an iterator by value</code>

However, in more complex scenarios, the distinction can be less apparent. Consider the following examples:

• Case 1: Reference or Copy?

<code class="cpp">const std::shared_ptr<Foo>&amp; get_foo();
auto p = get_foo(); // Copy or reference?</code>

In this case, "auto" deduces the type from the return type of get_foo() function, which is a reference to a std::shared_ptr. Since the declaration of p uses a single ampersand (&), it resolves to a copy, not a reference.

• Case 2: Static Variable - Copy or Reference?

<code class="cpp">static std::shared_ptr<Foo> s_foo;
auto sp = s_foo; // Copy or reference?</code>

Here, "auto" deduces the type from the declaration of s_foo, which is a static std::shared_ptr. Since there is no ampersand in the declaration of sp, it again resolves to a copy.

• Case 3: Looping Over a Container - Copy for Each Iteration?

<code class="cpp">std::vector<std::shared_ptr<Foo>> c;
for (auto foo: c) { // Copy for every loop iteration?</code>

In this case, "auto" deduces the type from the iterator type of the vector. The iterator returns std::shared_ptr, which is a reference type. However, since the declaration of foo uses an ampersand (&), it resolves to the dereferenced value, which is a copy of the std::shared_ptr for each loop iteration.

Conclusion:

The rule for "auto" type deduction is straightforward: it follows the declaration of the variable itself. To resolve to a reference, use an ampersand in the declaration (auto &ref = ...). Otherwise, "auto" will deduce a value type.

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