How to initialize std::vector from C-style array?

c++ arrays vector stl

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?

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.

Pavel Minaev

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

w_.assign(w, w + len);

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.

The quick solution could be std::vector w_(w,w+len);

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

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.

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

Mark B

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);
}

In my case, the assignment will happen repeatedly.

Mugurel

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.

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

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.

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).

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.

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

Community

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);
}

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

iambr

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

Janusz Lenar

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.

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

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

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

How to initialize std::vector from C-style array?

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