How Can I Safely Reuse a Moved Container in C ?

Moving Ownership and Reusing Containers in C

In C , when an object is moved (using the std::move function), its ownership is transferred to the new variable. This means that the original variable becomes empty and contains no valid data.

One common question that arises when working with moved containers is how to reuse them. The question arises: "What is the correct way to reuse a moved container?"

Understanding the "Valid but Unspecified State"

According to the C 0x standard draft, an object after move is in a "valid but unspecified state." This means that the object meets its invariants, but its internal state is not guaranteed to be consistent.

Option 1: Do Nothing

One option is to do nothing and assume the object is still valid. However, this approach is not recommended as it can lead to undefined behavior if the container's internal state is not consistent.

Option 2: "Reset" with clear

Another option is to use the clear method to "reset" the container and make it empty. This approach ensures the container's internal state is consistent and allows it to be reused safely.

Option 3: Reinitialize with Default Constructor

A final option is to simply reinitialize the container using its default constructor. This approach also ensures the container's internal state is consistent and can be used again.

Preferred Approach

The preferred approach for reusing a moved container is to use the clear method. This approach is both safe and efficient, and it avoids the potential pitfalls associated with the other options.

Example

Consider the following code:

std::vector<int> container;
container.push_back(1);
auto container2 = std::move(container);

//container2.clear(); Reset
container = std::vector<int>(); // Reinitialize

container.push_back(2);
assert(container.size() == 1 && container.front() == 2);

By using the clear method to reset container2, we ensure that its internal state is consistent and can be safely reused. As a result, the subsequent push_back operation correctly adds a new element to the container.

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