可变大小的char数组，同时最大限度地减少了对new的调用？

Question

I need a char array that will dynamically change in size. I do not know how big it can get so preallocating is not an option. It could never get any bigger than 20 bytes 1 time, the next time it may get up to 5kb...

I want the allocation to be like a std vector.

I thought of using a std vector < char > but all those push backs seem like they waste time:

strVec.clear();
for(size_t i = 0; i < varLen; ++i)
{
   strVec.push_back(0);
}

Is this the best I can do or is there a way to add a bunch of items to a vector at once? Or maybe a better way to do this.

Thanks

Answer 1

std::vector不会在每次调用push_back时分配内存，仅在大小变得大于容量时才分配

Answer 2

First, don't optimize until you've profiled your code and determined that there is a bottleneck. Consider the costs to readability, accessibility, and maintainability by doing something clever. Make sure any plan you take won't preclude you from working with Unicode in future. Still here? Alright.

As others have mentioned, vectors reserve more memory than they use initially, and push_back usually is very cheap.

There are cases when using push_back reallocates memory more than is necessary, however. For example, one million calls to myvector.push_back() might trigger 10 or 20 reallocations of myvector. On the other hand, inserting into a vector at its end will cause at most 1 reallocation of myvector*. I generally prefer the insertion idiom to the reserve / push_back idiom for both speed and readability reasons.

myvector.insert(myvector.end(), inputBegin, inputEnd)

If you do not know the size of your string in advance and cannot tolerate the hiccups caused by reallocations, perhaps because of hard real-time constraints, then maybe you should use a linked list. A linked list will have consistent performance at the price of much worse average performance.

If all of this isn't enough for your purposes, consider other data structures such as a rope or post back with more specifics about your case.

• From Scott Meyer's Effective STL, IIRC

Answer 3

You can use the resize member function to add a bunch. However, I would not expect that push_back would be slow, especially if the vector's internal capacity is already non-trivial.

Answer 4

Is this the best I can do or is there a way to add a bunch of items to a vector at once? Or maybe a better way to do this.

push_back isn't very slow, it just compares the size to the current capacity and reallocates if necessary. The comparison may work out to essentially zero time because of branch prediction and superscalar execution on the CPU. The reallocation is performed O(log N) times, so the vector uses up to twice as much memory as needed but time spent on reallocation seldom adds up to anything.

To insert several items at once, use insert . There are a few overloads, the only trick is that you need to explicitly pass end .

my_vec.insert( my_vec.end(), num_to_add, initial_value );
my_vec.insert( my_vec.end(), first, last ); // iterators or pointers

For the second form, you could put the values in an array first and then copy the array to the end of the vector. But this might add as much complexity as it removes. That's how it goes with micro-optimization. Only attempt to optimize if you know there's a measurable gain to be had.