Why isn't the C 11 move constructor called when instantiating an object with another object?

C 11 Move Constructor Not Called, Default Constructor Preferred

Question:

In C 11, when instantiating an object via another object, why might the move constructor not be called when it should be?

Answer:

Upon instantiation, the compiler may occasionally apply a technique known as copy elision. Copy elision allows the direct construction of a temporary object into the target it would be copied or moved into, bypassing the copy or move constructor/destructor pair.

The standard allows copy elision in the following circumstances:

• Return Statements: Non-volatile automatic objects with the same cv-unqualified type as the function return type can be constructed directly into the return value.
• Throw Expressions: Non-volatile automatic objects whose scope does not extend beyond the innermost enclosing try-block can be constructed directly into the exception object.
• Temporary Objects: Temporary objects that have not been bound to a reference can be constructed directly into objects with the same cv-unqualified type.
• Exception Handlers: Exception declarations of an exception handler can be treated as aliases for the exception object, omitting the copy or move operation.

In the given example, when instantiating z with X("test"), copy elision occurs because it is considered a temporary object that has not been bound to a reference. Consequently, it is constructed directly into z, bypassing the move constructor and constructing z using the default constructor instead.

Explicitly calling move(X("test")) prevents copy elision and forces the use of the move constructor, as observed in the modified output.

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