Question

I have a struct that looks like this:

struct queue_item_t {
    int id;
    int size;
    std::string content;
};

I have a std::vector< queue_item_t > which is populated with many of these from a database query.

When each item is processed, a file is read from disk and its contents put into the content string member. The item is processed (content is parsed) and I execute .clear() on the string so as not to eat up all my memory.

However, this doesn’t seem to free the memory. I have hundreds of thousands of items being processed and eventually, the memory usage will rise beyond what’s available and the application is killed by Linux with “Out-Of-Memory” as reason.

How do I free the memory used by these strings?

Answer 1

std::string and std::vector do not change container capacity (=> do not release container memory) in clear(). The below trick with temporary object should be used whenever compaction is required (normally it is not required).

my_queue_item.content.clear(); // clear
std::string(my_queue_item.content).swap(my_queue_item.content); // compact

above code can be made simpler when cleanup+compaction are requried:

std::string().swap(my_queue_item.content);

Some string implementations are copy-on-write. Such string, when referred from more than one place, would re-allocate memory on write.