Benefits of vector over string?

问题:

This question is related to, but not quite the same as, this question.

Are there any benefits to using std::vector<char> instead of std::string to hold arbitrary binary data, aside from readability-related issues?

i.e. Are there any tasks which are easier/more efficient/better to perform with a vector compared to a string?

回答1:

Aside from readability (which should not be underestimated) I can think of a couple of minor performance/memory issues with using std::string over std::vector:

• Some modern std::string implementations use the small string optimization. If you are storing data that's larger than the string's internal buffer, it becomes a pessimization, reducing the efficiency of copying, moving, and swap¹ and increasing the sizeof() for no benefit.

• An efficient std::string implementation will always allocate at least 1 more byte than the current size for storing a terminating null (not doing so requires extra logic in operator[] to cope with str[size()]).

I should stress that both of these issues are very minor; the performance cost of them will more than likely be lost in the background noise. But you did ask.

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¹Those operations require branching on size() if the small string optimization is being used, whereas they don't in a good std::vector implementation.

回答2:

Beyond readability, and ensuring another maintainer does not confuse the purpose of the std::string, there is not a lot of difference in function. You could of course consider char*/malloc as well, if efficiency is the only consideration.

One potential issue I can think of:

std::string defaults to storing <char>. If you later needed to handle another type (e.g. unsigned short) you might need to either:

• Create your own typedef std::basic_string<unsigned short> (which moves you away from normal std::string handling)
• Tentatively apply some reinterpret_cast logic in a setter.

With a vector you could simply change the container to a std::vector<unsigned short>.

回答3:

I think the only benefit you would gain from doing that would the ease of incrementing over the std::vector of characters, but even that can be done with an std::string.

You have to remember that even though std::string seems like an object, it can be accessed like an array, so even accessing specific parts of a string can be done without the use of a std::vector

回答4:

Ideally one would use vector<unsigned char> to store arbitrary binary data - but I think you already knew this - as you referred to the old question.

Other than that, using vector would definitely be more memory efficient, as string would add a terminating Nul character. Performance might also improve as the allocation mechanism is different for both - vectors guarantee contiguous memory!

Besides that, using a string would not be correct, as callers/users might inadvertently invoking some of the string methods, which could be a disaster.

回答5:

Yes, vector<char> indeed does have more capabilities over string.

Unlike string, vector<char> is guaranteed to preserve iterators, references, etc. during a swap operation. See: May std::vector make use of small buffer optimization?