How Does \"auto\" Determine Value vs. Reference Types in C 11?

Type Deduction Semantics of "auto" in C 11

In C 11, the "auto" keyword infers the type of a variable from the type of its initializer. However, determining whether "auto" resolves to a value or a reference can sometimes be ambiguous.

Value vs. Reference

The key rule for type deduction with "auto" is that the deduced type is equivalent to the declared type.

• If the declared type is a value type, "auto" will resolve to a value.
• If the declared type is a reference type, "auto" will resolve to a reference.

Examples

• auto i = v.begin(); - Since v.begin() returns an iterator by value, "auto" resolves to a value.
• const std::shared_ptr& get_foo(); - The type of the function is a reference, so "auto" will resolve to a reference.
• static std::shared_ptr s_foo; - The type of the variable is a pointer, so "auto" will resolve to a value.
• std::vector> c; - The type of the container is a vector of pointers, so "auto" will resolve to a value.

Type Deduction

The following example demonstrates the type deduction behavior:

<code class="cpp">int i = 5;
auto a1 = i;    // value
auto &a2 = i;  // reference</code>

In this example, "a1" is of type int (value), while "a2" is of type int& (reference).

Conclusion

Understanding the type deduction semantics of "auto" is crucial to write correct and efficient C code. By adhering to the rule of "auto" resolving to the declared type, developers can accurately infer variable types and leverage the benefits of this feature.

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