C++11 introduced a new member function called shrink_to_fit() for the three sequence containers vector, deque, and string. Before shrink_to_fit() there was no member to reduce capacity on these containers, not even erase(). In fact, the standard made it very unlikely that any implementation would reduce capacity with any call. The standard provides the guarantee that if reserve() is called, then any subsequence addition to the container will not invalidate iterators (until an addition increases the size of the container above the reserve() point.

Since this guarantee is not relaxed even if the user calls erase(), a implementation that chose to reduce capacity would be required to track whether or not reserve had previously been called. It would be theoretically possible for an implementation to add this extra state to the container implementation, but in practice no implementation is going to do that.

Perhaps this lack of attention to minimizing capacity isn’t surprising. In most modern code, our concern is usually time performance rather than space requirements. But C++ is the language of choice for demanding situations and sometimes this means minimizing space requirements.

So what can users do if they want to reduce the capacity of a container? Is this not possible? It turns out that it is possible. In Item 17 of Scott Meyer’s Effective STL, Scott explains that there is a “Swap Trick” that we can use to reduce capacity.

Here is the quick synopsis of the Swap trick:

Container(begin(c), end(c)).swap(c);

where “c” is a container whose capacity we want to reduce and Container is it’s type.

What is happening is that we are creating a temporary container as a copy of our original container using the iterator range constructor and then swapping the contents of the our temporary container with our original container. This works because most implementations will implement the iterator range constructor to not make the capacity any larger than necessary. The reserve() guarantee is swapped to the other container with the call to swap().

In practice we’d almost certainly want to write this something like this:

if (c.capacity() > c.size()) { Container(begin(c), end(c)).swap(c); }

or:

if (c.capacity() > (c.size() + some_value)) { Container(begin(c), end(c)).swap(c); }

Because this trick involves copying all the items in the container, we don’t want to do it unless it saves enough space to be worthwhile. (In C++11 we could use moves instead of copies. This would make the technique less expensive, but we really only want to do this if the moves are no_except. This is a beyond the scope of what I want to discuss in this post.)

Meyers’ Swap Trick was state of the art, at least until C++11 where we can now do this:

if (c.capacity() > c.size()) { c.shrink_to_fit); }

assuming type of c is vector, deque, or string. For vector and string the standard says “… request to reduce capacity() to size().” (We’ll consider the ellipse shortly.) For deque the standard says “… request to reduce memory use.”

Why is deque’s description different than vector and string? Because deque doesn’t have a capacity() member function. The point of the capacity member function is tell us how much we can grow the number of items in the container before an addition would invalidate iterators. This concept doesn’t make much sense for deque, so there is no such call as deque::capacity().

So let’s look at what I left out just now when I quoted the standard. What it really says for vector and string is “shrink_to_fit is a non-binding request to reduce capacity() to size().” The emphasis is mine. The standard has made shrink_to_fit() non-binding. (Also true for deque.) It can be implemented as a no-op!

Why would the committee do such a thing? The standard gives an answer: “[Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ]” What this means is that the committee knows that there are implementations where doing the right thing might not result in size() == capacity().

Consider the “short string” optimization. A string is an allocated array and a straight-forward implementation would allocation an array even for a string that was only one character long. One character long strings are not particularly common, but short strings are. So an library might choose to implement the string class so that the class itself has (data member) space for “short” strings so that no allocation is necessary until the size of the string is no longer “short” for some value of “short.” Given this implementation how would capacity() be implemented? In particular would capacity() ever return a value less than the “short” string limit? No because we can always expand the string to the “short” string limit without invalidating iterators (assuming the current size() is less than this limit).

This means that if we currently have a string whose size() is less than the “short” string limit, a “request to reduce capacity() to size()” is going to fail. So the standard doesn’t want to say that the implementation must reduce capacity() to size().

But in my opinion the committee has let us down. While I’m all for giving implementers latitude for their implementations, I think the committee has underspecified this call. It is acceptable to say that the implementation need not reduce capacity() all the way to size(), but it should say that it will reduce capacity at least as far as the Swap Trick would have.

Since the Swap Trick is potentially expensive in time, it is likely only used in situations where users are very serious about reducing excess capacity. Given this, and faced with the fact that shrink_to_fit() may be a no-op, I think conscientious coders will choose to skip the new shrink_to_fit() and continue to use the Swap Trick.

And that is a shame. With shrink_to_fit() the user is very explicitly stating the desired outcome, so this has the potential to allow implementers to exploit interesting optimizations. This should be better than the somewhat obscure looking Swap Trick. But clarity and interesting optimizations aren’t of much use if users don’t use the new call. And they may avoid it because the old way gives them guarantees that the committee didn’t see fit to apply to the new call.

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