dynamically-sized text object with a copy constructor, a trivial assignment operator, and a trivial destructor

问题

I've been shown that a std::string cannot be inserted into a boost::lockfree::queue.

boost::lockfree::queue is too valuable to abandon, so I think I could use very large, fixed length chars to pass the data according to the requirements (assuming that even satifies since I'm having trouble learning about how to satisfy these requirements), but that will eat up memory if I want large messages.

Does a dynamically-sized text object with a copy constructor, a trivial assignment operator, and a trivial destructor exist? If so, where? If not, please outline how to manifest one.

回答1:

A dynamically-size type with a trivial copy ctor/dtor is not possible. There are two solutions to your problem, use a fixed sized type, or store pointers in the queue:

boost::lockfree::queue<std::string*> queue(some_size);
// push on via new
queue.push(new std::string("blah"));
// pop and delete
std::string* ptr;
if(queue.pop(ptr))
{
   delete ptr;
}

回答2:

Does a dynamically-sized text object with a copy constructor, a trivial assignment operator, and a trivial destructor exist?

Dynamically sized, no. For something to have a trivial destructor, it requires that the destructor of the object is implicit (or defaulted), and any non-static member objects also have implicit (or defaulted) destructors. Since anything that is dynamically allocated will require a delete [] somewhere along the line in a destructor, you cannot have this constraint satisfied.

To expand upon the above, consider a (very cut down) example of what happens in std::string:

namespace std
{
    // Ignoring templates and std::basic_string for simplicity
    class string 
    {
    private:
        char* internal_;
        // Other fields

    public:
        string(const char* str)
          : internal_(new char[strlen(str) + 1])
        { }

    };
}

Consider what would happen if we left the destructor as default: it would destroy the stack-allocated char * (that is, the pointer itself, not what it points to). This would cause a memory leak, as we now have allocated space that has no references and hence can never be freed. So we need to declare a destructor:

~string()
{
    delete[] internal_;
}

However, by doing this, the destructor becomes user-defined and is therefore non-trivial.

This will be a problem with anything that is dynamically allocated. Note that we cannot fix this by using something like a shared_ptr or a vector<char> as a member variable; even though they may be stack allocated in our class, underneath, they are simply taking care of the memory management for us: somewhere along the line with these, there is a new [] and corresponding delete [], hence they will have non-trivial destructors.

To satisfy this, you'll need to use a stack allocated char array. That means no dynamic allocation, and therefore a fixed size.

来源：https://stackoverflow.com/questions/18710199/dynamically-sized-text-object-with-a-copy-constructor-a-trivial-assignment-oper