向量的初始化

Question

I saw different ways to initialize vectors from an array (c-style or std::array) in C++11. Suppose I have an array like this : uint8_t arr[5000] or like this : std::array<uint8_t, 5000> arr; as an example.

If I want the 5000 values from arr, I can initialize it these ways :

1. std::vector<uint8_t> vect(arr, arr + 5000);

2. std::vector<uint8_t> vect(std::begin(arr), std::end(arr));

3. std::vector<uint8_t> vect(arr.begin(), arr.end()); This one apply if arr is an std::array<uint8_t, 5000> )

According to this answer , option 3 is better than option 1 and 2. It's better to use member function of the container instead of non-member functions.

I always saw the first option for c-style arrays. Is the option 1 is exactly the same as the option 2 ? What's the subtleties behind that ?

Thanks for your help.

Answer 1

The preferred way is to put the values directly into the vector to start with, rather than putting them into an array, then using it to initialize the vector.

Barring that, the version using the free functions std::begin and std::end is generally preferable over the alternatives. It works whether the source is an array, std::vector , std::array , or one of the other containers such as std::list , or std::set .

The latter is obviously of greatest important in generic code, so it can be independent of the type of the source container. Even otherwise, however, being able to use identical syntax for essentially all types of input makes it one more thing you can recognize as a high-level pattern, so you don't need to spend brain-cycles on figuring out what it's doing.

There are two obvious disadvantages:

1. You might need the code portable to an older compiler that doesn't implement begin and end as free functions. In my opinion, the best cure for this is to implement them yourself if needed (minimal effort).
2. You have an existing code base that makes heavy use of container.begin() and container.end() . Modifying a large existing code base to use the free functions may not be practical, and having a mixture of x.begin() and begin(x) won't help readability.