Exception guarantees when moving object into std::vector

Question

If memory allocation fails while moving a object into a std::vector, and bad_alloc is thrown, does std::vector guarantee that the moved from object is unaltered/still valid ?

For example:

std::string str = "Hello, World!";
std::vector<std::string> vec;

vec.emplace_back(std::move(str));
/* Is str still valid and unaltered if the previous line throws? */

Answer 1

This is covered in [container.requirements.general]/11.2

if an exception is thrown by a push_­back(), push_­front(), emplace_­back(), or emplace_­front() function, that function has no effects.

So, you will not have a moved from object in the event of an exception.

This only covers if the vector throws. We need to look at [vector.modifiers]/1 to see what happens if the move constructor itself throws:

If an exception is thrown while inserting a single element at the end and T is Cpp17CopyInsertable or is_­nothrow_­move_­constructible_­v<T> is true, there are no effects. Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.

^{emphasis mine}

And in that case the behavior is unspecified.

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In your case std::string has a noexcpet move constructor so you will have a valid object if an exception throws.