How Does Returning a Unique Pointer by Value Avoid Copy Construction in C ?

Return by Value in Unique Pointers: Understanding the Exception

Problem:

Unique pointers (std::unique_ptr) in C strictly adhere to move semantics, disallowing copy construction. However, it is possible to return a unique pointer by value from a function and assign the returned value without invoking the copy constructor. This action raises questions about how this seemingly paradoxical behavior is permitted.

Question:

Is there a specific clause in the language specification that permits this exception?

Answer:

Yes. As noted in C 11's §34 and §35, the compiler is authorized to perform "copy elision" in certain scenarios. Here is the relevant excerpt:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object [...]. This elision of copy/move operations, called copy elision, is permitted [...] in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object with the same cv-unqualified type as the function return type [...].

In essence, this means that if the return value is a non-volatile, automatic object that matches the return type, the compiler can skip the copy/move construction.

Additionally, as per the specification:

When the criteria for elision of a copy operation are met and the object to be copied is designated by an lvalue, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.

This implies that when returning an lvalue (named object) by move, the compiler will attempt to resolve the constructor selection as if it were an rvalue (temporary object).

Implementation Details:

This optimization is implemented through compiler techniques. In cases where copy elision is permitted, the compiler directly creates the returned object in the designated memory location without undergoing copy construction. This approach ensures that the returned object is unique and avoids unnecessary object creation and destruction.

It's important to note that this behavior is specific to C 0x, and in previous versions of C , returning a unique pointer by value would typically result in undefined behavior or compiler errors.

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