Why Can I Return a `std::unique_ptr` Without `std::move`?

Why Returning a std::unique_ptr Without std::move is Permitted

Although std::unique_ptr prohibits copy construction, return statements featuring unique_ptrs are exempt. This behavior aligns with the C language specification, particularly clauses 12.8 §34 and §35.

When specific conditions are satisfied, implementations may omit copy/move construction for class objects. This optimization, known as copy elision, is allowed in return statements for class return types within functions. The exception applies to expressions involving a non-volatile automatic object whose type matches the function return type.

Moreover, when copy elision occurs for an lvalue object, overload resolution for the copy constructor proceeds as if the object were designated by an rvalue.

Compiler Implementation

This exception is handled within the compiler. Compilers leverage language specifications to recognize and apply elision techniques, optimizing code by eliminating unnecessary operations.

Named Value Return

It's noteworthy that returning by value is the optimal option in such scenarios. Even without elision, named values in return statements are handled as rvalues, incurring minimal overhead.

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