How to assign a std::vector using a C-style array?

Question

What is the cheapest way to initialize a std::vector from a C-style array?

Example: In the following class, I have a vector, but due to outside restrictions, the data will be passed in as C-style array:

class Foo {
  std::vector<double> w_;
public:
  void set_data(double* w, int len){
   // how to cheaply initialize the std::vector?
}

Obviously, I can call w_.resize() and then loop over the elements, or call std::copy(). Are there any better methods?

Sacerdotal

The crux of the problem is that there is no way for the vector to know if the same allocator was used to create your C-style array. As such the vector must allocate memory using its own allocator. Otherwise it could simply swap out the underlying array and replace it with your array. – Thunderstone

Answer 1

Don't forget that you can treat pointers as iterators:

w_.assign(w, w + len);

Weep

Is there a performance difference between std::copy(w, w + len, w_.begin()) and your assign solution? – Sacerdotal

Oh, I guess the difference is that std::copy will not resize the array. – Sacerdotal

I don't know if assign is smart enough to compute how much w is distant from w+len (a generic iterator AFAIK does not provide a quick way to do this), so you may enhance a little the performances putting a w_.reserve(len) before that statement; in the worst case you gain nothing. In this way it should have more or less the performance of resize+copy. – Sluggard

It's a quality of implementation issue. Since iterators have tags that specify their categories, an implementation of assign is certainly free to use them to optimize; at least in VC++, it does indeed do just that. – Weep

The quick solution could be std::vector<double> w_(w,w+len); – Avlona

This copies elements to a newly created storage for 'w_'; 'w_.data' will not point to 'w'. You still have to deallocate 'w'. There's no ownership transfer – Grogshop

Sure, but this is as good as you can get with a vector. It does not provide any facility to attach to an existing buffer. – Weep

Is this safe to do when when w+len points to the element past the end of the array, as in, when you want to use this method to take the last 10 elements of a 100 element array? In VS2013 I got a debug assertion that the source array had been accessed out of bounds, which made me cautious, and I ended up using a more C-style approach. – Haematoxylin

If it's one element past the end, it should be okay (just as v.end() is an iterator pointing one past the end with vector in a similar case). If you do get an assertion, then something is off elsewhere. – Weep

Also if you want to statically allocate in C++11 use initializer_list like below vector<double> w_{1.2, 2.3, 4.3}; – Gaseous

Just quickly, will this deallocate the array memory when the vector goes out of scope? – Dwan

Answer 2

You use the word initialize so it's unclear if this is one-time assignment or can happen multiple times.

If you just need a one time initialization, you can put it in the constructor and use the two iterator vector constructor:

Foo::Foo(double* w, int len) : w_(w, w + len) { }

Otherwise use assign as previously suggested:

void set_data(double* w, int len)
{
    w_.assign(w, w + len);
}

Marionmarionette

In my case, the assignment will happen repeatedly. – Sacerdotal

Answer 3

The quick generic answer:

std::vector<double> vec(carray,carray+carray_size); 

---

or question specific:

std::vector<double> w_(w,w+len); 

---

based on above: Don't forget that you can treat pointers as iterators

Diet

Answer 4

Well, Pavel was close, but there's even a more simple and elegant solution to initialize a sequential container from a c style array.

In your case:

w_ (array, std::end(array))

• array will get us a pointer to the beginning of the array (didn't catch it's name),
• std::end(array) will get us an iterator to the end of the array.

Merlenemerlin

What includes/version of C++ does this require? – Wiatt

This is one of the constructors of std::vector from at least c++98 onwards.... It's called 'range constructor'. cplusplus.com/reference/vector/vector/vector Try it. – Merlenemerlin

More independent version is: w_ (std::begin(array), std::end(array)); (In the future you can to change a C array for a C++ container). – Lack

Mind you, this only works if you have a real array (which usually means you're copying from a global or local (declared in current function) array). In the OP's case, he's receiving a pointer and a length, and because it's not templated on the length, they can't change to receiving a pointer to a sized array or anything, so std::end won't work. – Ashton

vector does not overload operator(), so this won't compile. std::end being called on a pointer is no use either (the question asks to assign a vector from a pointer and a separate length variable). It would improve your answer to show more context about what you are trying to suggest – Todo

Answer 5

You can 'learn' the size of the array automatically:

template<typename T, size_t N>
void set_data(const T (&w)[N]){
    w_.assign(w, w+N);
}

Hopefully, you can change the interface to set_data as above. It still accepts a C-style array as its first argument. It just happens to take it by reference.

---

How it works

[ Update: See here for a more comprehensive discussion on learning the size ]

Here is a more general solution:

template<typename T, size_t N>
void copy_from_array(vector<T> &target_vector, const T (&source_array)[N]) {
    target_vector.assign(source_array, source_array+N);
}

This works because the array is being passed as a reference-to-an-array. In C/C++, you cannot pass an array as a function, instead it will decay to a pointer and you lose the size. But in C++, you can pass a reference to the array.

Passing an array by reference requires the types to match up exactly. The size of an array is part of its type. This means we can use the template parameter N to learn the size for us.

It might be even simpler to have this function which returns a vector. With appropriate compiler optimizations in effect, this should be faster than it looks.

template<typename T, size_t N>
vector<T> convert_array_to_vector(const T (&source_array)[N]) {
    return vector<T>(source_array, source_array+N);
}

Lachesis

In the last sample, return { begin(source_array), end(source_array) }; is also possible – Todo

Answer 6

std::vector<double>::assign is the way to go, because it's little code. But how does it work, actually? Doesnt't it resize and then copy? In MS implementation of STL I am using it does exactly so.

I'm afraid there's no faster way to implement (re)initializing your std::vector.

Cheboksary

what if data to be shared between the vector and an array? Do we need to copy anything in this case? – Wiatt

is that an answer or a question? what does it bring to the already existing answers? – Trod

@Jean-FrançoisFabre and what does your comment bring? ;) true, it's a poor answer given ages ago. – Cheboksary