What Can You Do with a Moved-From Object in C ?

What Operations are Permitted on Moved-From Objects?

The C standard defines the behavior of objects that have been moved from. This knowledge is crucial for understanding how certain functions, such as std::swap, work.

Standard Definition

According to the standard (section 17.6.5.15), objects from C standard library types can be moved and are subsequently placed in a "valid but unspecified state."

Implication for Operations

When an object is in an unspecified state, you can perform any operation that does not have preconditions. Preconditionless operations include:

• Destruction
• Assignment
• Constant observers (e.g., get, empty, size)

However, operations with preconditions generally cannot be performed without further checking. For example:

• Dereference
• pop_back

Example: std::swap

The std::swap function, as shown in the original question, requires assignment to moved-from objects. This is possible because assignment is a preconditionless operation.

Additional Note: T c = std::move(a) vs. T c(std::move(a))

In the std::swap implementation, a direct move assignment T c = std::move(a) is used instead of a copy constructor T c(std::move(a)). This is done to avoid an unnecessary copy of a, which would result in a performance penalty.

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