How to initialize std vector from 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 lt double gt  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

User · Accepted Answer

Don t forget that you can treat pointers as iterators   w  assign w  w   len

User · Answer

You can  learn  the size of the array automatically   template lt typename T  size t N gt  void set data const T   amp 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 lt typename T  size t N gt  void copy from array vector lt T gt   amp target vector  const T   amp 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 lt typename T  size t N gt  vector lt T gt  convert array to vector const T   amp source array  N         return vector lt T gt  source array  source array N

User · Answer

The quick generic answer   std  vector lt double gt  vec carray carray carray size        or question specific     std  vector lt double gt  w  w w len        based on above  Don t forget that you can treat pointers as iterators

User · Answer

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

User · Answer

std  vector lt double gt   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

User · Answer

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

[c++] How to initialize std::vector from C-style array?

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