Initialising boost::array after declaration

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We can initialise a boost or std::array using the following syntax:

array<int,5> b = {1, 2, 3, 4, 5};

This is fine if 'b' is a local variable. How about if 'b' is a class member?

b = {1, 2, 3, 4, 5}; // Error: expected an expression
b = array<int,5>{1, 2, 3, 4, 5}; // Error: type name is not allowed 
b = array<int,5>({1, 2, 3, 4, 5}); // Error: expected an expression
b = array<int,5>(1, 2, 3, 4, 5); // Error: no suitable constructor exists to convert from "int" to "array<int, 5>"

Do you really have to do this:

array<int,5> c = {1, 2, 3, 4, 5};

b = c;

Which seems a little wasteful as it creates 'c', initialises it, and then copies it into b before destroying 'c'.

You can create them as in your question which will create a temporary on the stack and then copy it across to your instance (which may be a little wasteful), or you can use static variables and an initialisation list in which any compiler worth its salt will just initialise the appropriate members of the class:

class temp
{
   public:
   temp():b(b_default) {}
   array<int, 5> b;
   static array<int, 5> b_default;
};
array<int, 5> temp::b_default = {1,2,3,4,5};

This way is probably the 'cleanest' way however: (again, a single copy for all decent compilers)

class temp
{
   public:
   temp() 
   {
    static const array<int, 5> b_default = {1,2,3,4,5};

    b = b_default;
   }
   array<int, 5> b;
   static const array<int, 5> b_default;
};

c++ - Initialising boost::array after declaration, You can create them as in your question which will create a temporary on the stack and then copy it across to your instance (which may be a little wasteful),� void printArray(const std::array<int, 5> &n) - const is used here to prevent the compiler from making a copy of the array and this enhances the performance. The passed array will be n in this function as &n is the parameter of the function 'printArray'.

You can also initialize data members at the point of declaration:

struct Foo
{
  array<int,5> b = {1, 2, 3, 4, 5};
};

Alternatively, you can also use the constructor initialization list

struct Foo
{
  Foo() : b{1, 2, 3, 4, 5} {}
  array<int,5> b;
};

Note, all your statements involving b = X are not initializations, they are assignments.

Edit: This kind of initialization is not possible in C++03, but you could achieve something similar by calling a function that returns an suitable initialized array:

boost::array<int, 5> make_array()
{
  // trivial example, but you can do more complicated stuff here
  boost::array<int, 5> a = {{1, 2, 3, 4, 5}};
  return a;
}

then

Foo() : b(make_array()) {}

Value Initialized, In a C++ array declaration, the array size is specified after the each element for (size_t i = 0; i < size; i++) { std::cout << numbers[i] << " "; } You can initialize an array in a loop, one element at a time, or in a single statement. When an array is initialized with a brace-enclosed list of initializers, the first initializer in the list initializes the array element at index zero (unless a designator is specified) (since C99), and each subsequent initializer without a designator (since C99) initializes the array element at index one greater than the one initialized by the previous initializer.

You may use the following:

struct S
{
  std::array<int, 5> b = {{1, 2, 3, 4, 5}};
};

Note the double {

Boost.Assignment Documentation - 1.57.0, We can think of boost::array as the container std::vector with the declaration boost::array<int, 5> c = { 1,3,4,2,5 };//we can initialize the array //while endl; cout << "array c after being passed to fun";//note that there will //be no� How to declare an array in C? Data_type array_name[size_of_array]; For example, float num[10]; Below are some of the different ways in which all elements of an array can be initialized to the same value: Initializer List: To initialize an array in C with the same value, the naive way is to provide an initializer list. We use this with small arrays.

Personnaly, I solve this kind of problem by creating my own initialization functions. Here, with preprocessor (in order to stay compatible with C++03), but it can be done with variadic templates (>= c++11).

#define NMAX_MAKE_ARRAY_PARAMETERS 30
#define MAKE_ARRAY_EDIT(z, n, data) \
  a__[n] = x__ ## n;
#define MAKE_ARRAY_DEFINITION(z, n, data) \
  template <typename T> boost::array<T,n> make_array(BOOST_PP_ENUM_PARAMS(n, const T& x__)) \
  { \
    boost::array<T,n> a__; \
    BOOST_PP_REPEAT(n, MAKE_ARRAY_EDIT, 0) \
    return a__; \
  }
#define MAKE_ARRAY_DEFINITIONS(n) \
  BOOST_PP_REPEAT(BOOST_PP_INC(n), MAKE_ARRAY_DEFINITION, 0)
MAKE_ARRAY_DEFINITIONS(NMAX_MAKE_ARRAY_PARAMETERS)

With this code included, I can easily create arrays having size between 0 and 30 :

boost::array<double,5> a = make_array<double>(1.0, 2.2, 3.4, 4.6, 5.8);
a = make_array<double>(0.0, 0.1, 0.2, 0.3, 0.4);

Arrays (C++), Different ways to Initialize all members of an array to the same value in C � Different ways to declare variable as constant in C and C++ � Can we� A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions.

C++ Boost Array | C++, last overwrite the first. Having a trailing comma after the last defined array entry , while unusual, is a valid syntax. $i++; // increase counter } // end subarray loop During initialization php parses every value as expression. One can declare� Initialization means telling the system about the size of memory needed to hold the variable as we are initially assigning something. We can initialize our variable according to our further uses. For example, each year age is increasing below.

Different ways to initialize a variable in C/C++, std::list provides various overloaded constructors for creation and initialization of list. Let's see different ways to create & initialize a std::list in C++ i.e.. Creating an Empty List in C++. std::list has a default constructor that will create an empty list i.e. Initializing a std::list with a std::vector or an Array. std::list� Systemverilog Dynamic Array examples array resize array of classes constraints initialization size copy dynamic array is one dimension of an unpacked array

array - Manual, After considering different names, we decided to name this class simply array. Note that boost::array — STL compliant container wrapper for arrays of constant size then each remaining element gets default-initialized (thus, it has a defined. Initialization of string array. String array can be initialized using the new keyword. We cannot initialize string array without specifying it’s the size. There are two ways to initialize a string array. 1. At the time of declaration: string[] variable_name = new string[size]; 2. After declaration: string [] variable_name;

Comments
  • State the version of the compiler.
  • I don't think the version of the compiler will help, but it supports C++ 2003 standard.
  • So you use c++03 (which is different than c++0x) ?
  • Maybe I don't know what c++0x actually is, but yes, it's 2003. All these different versions confuse me.
  • c++0x is a place-holder for "the next standard" before C++11 got ratified. So is an incomplete implementation of the best interpretation of the future standard as it was being developed. So the compiler version does actually matter, because the larger the version, the closer it comes to C++11.
  • You are still initializing then assigning to the data member. There's no need for that.
  • It generates exactly the same code as your answer, but without having to have a method for every single array I need to initialise (just an extra variable that gets optimised away), so I'd say it was the cleanest way here. If you have a better method, please post it.
  • Sorry, the variable is in the class and the compiler doesn't support data initialisation, so I have to do the init in the constructor. I guess they are assignments though, yes.
  • @Steve The variable is in the class here too. Anyway, the above is valid C++11. If you can't use C++11 you should state that clearly in the question.
  • I clearly said "I'm not using C++11". I've now removed that and added the tag "C++03" instead.
  • @Steve Well, you managed to get two people to give you C++11 answers, so obviously there was some confusion with the tags and/or the question.
  • Indeed. So are you saying it's impossible in C++03 ? If not, adding this to your question will get it accepted :)