How Should You Properly Reuse Moved Containers in C ?

Reusing Moved Containers: The Right Approach

The topic of reusing moved containers in C has sparked discussions. This article explores the correct method for handling such situations.

Consider the following code snippet:

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

// ver1: Do nothing
//container2.clear(); // ver2: "Reset"
container = std::vector<int>() // ver3: Reinitialize

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

According to the C 0x standard draft, ver3 appears to be the preferred approach since a moved object resides in "a valid but unspecified state." To clarify this concept, let's refer to the definition in the standard:

[defns.valid]
§17.3.26     valid but unspecified state

an object state that is not specified except that the object’s invariants are met and operations on the object behave as specified for its type

This means that the moved object is still alive and can be operated upon as usual, excluding actions with preconditions (unless the precondition is verified beforehand).

For instance, the clear() operation has no preconditions. By employing clear(), the container is reset to a defined state, enabling its subsequent utilization.

Consequently, the correct way to reuse a moved container is to perform a clear() operation, as demonstrated in ver2 in the code snippet. This approach ensures that the container is restored to a known and consistent state.

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