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I think if you come from the JVM / CLR world, you are so protected by the runtime’s patching up of references that it might not even occur to you that a (raw) pointer to a data structure’s internals can dangle after the data is moved around. The runtimes mentioned pause your code, move things around and even compact the heap and your references magically still point to what they did before!

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Except, it's not magic. It costs processing cycles, and latency, and invalidations of cache rows you were using, and memory bus traffic. Lots of all of them.

Most of the costs are not counted as runtime for your process, so benchmarks invariably appear to show GC as costing less overhead than it does. Among the costs, as with all the myriad varieties of caching done in modern systems, is that GC makes it hard to know the costs of design choices you make. Many of the costs are in making the caches less effective.

Well it's not so much being protected by patching up references, but more that C++ is (as far as I know) the only language that doesn't complain when you create a reference to an object that can move away.

Most languages either don't have the concept of classes or ensure that invalidating references is an explicit operation (such as calling the destructor).

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