How do I declare an array with a custom class

Question

I m trying to declare an array with a custom class  When I added a constructor to the class  my compiler complains that there s  No matching constructor for initialization of name 3      Here s my program    include  lt iostream gt   using namespace std   class name     public      string first      string last     name string a  string b       first   a      last   b          int main  int argc  const char   argv         const int howManyNames   3     name someName howManyNames      return 0      What can I do to make this run  and what am I doing wrong

User · Accepted Answer

You have to provide a default constructor. While you're at it, fix your other constructor, too:

class Name
{
public:
  Name() { }
  Name(string const & f, string const & l) : first(f), last(l) { }
  //...
};

Alternatively, you have to provide the initializers:

Name arr[3] { { "John", "Doe" }, { "Jane", "Smith" }, { "", "" } };

The latter is conceptually preferable, because there's no reason that your class should have a notion of a "default" state. In that case, you simply have to provide an appropriate initializer for every element of the array.

Objects in C++ can never be in an ill-defined state; if you think about this, everything should become very clear.

An alternative is to use a dynamic container, though this is different from what you asked for:

std::vector<Name> arr;
arr.reserve(3);  // morally "an uninitialized array", though it really isn't

arr.emplace_back("John", "Doe");
arr.emplace_back("Jane", "Smith");
arr.emplace_back("", "");

std::vector<Name> brr { { "ab", "cd" }, { "de", "fg" } }; // yet another way

User · Answer

In order to create an array of objects  the objects need a constructor that doesn t take any paramters  that creates a default form of the object  eg  with both strings empty   This is what the error message means  The compiler automatically generates a constructor which creates an empty object unless there are any other constructors   If it makes sense for the array elements to be created empty  in which case the members acquire their default values  in this case  empty strings   you should   -Write an empty constructor   class name     public      string first      string last     name         name string a  string b       first   a      last   b           -Or  if you don t need it  remove the existing constructor   If an  empty  version of your class makes no sense  there is no good solution to provide initialisation paramters to all the elements of the array at compile time  You can    Have a constructor create an empty version of the class anyway  and an init   function which does the real initialisation Use a vector  and on initialisation create the objects and insert them into the vector  either using vector  insert or a loop  and trust that not doing it at compile time doesn t matter  If the object can t be copied either  you can use an array vector of smart pointers to the object and allocate them on initialisation  If you can use C  11 I think     you can use initialiser lists to initialise a vector and intialise it  I m not sure if that works with any contructor or only if the object is created from a single value of another type   Eg        std  vector lt std  string gt  v      xyzzy    plugh    abracadabra

User · Answer

You need a parameterless constructor to be able to create an instance of your class  Your current constructor requires two input string parameters   Normally C   implies having such a constructor   default parameterless constructor  if there is no other constructor declared  By declaring your first constructor with two parameters you overwrite this default behaviour and now you have to declare this constructor explicitly   Here is the working code    include  lt iostream gt    include  lt string gt       lt -- you need this if you want to use string type  using namespace std    class name      public       string first       string last      name string a  string b        first   a       last   b           name         lt -- this is your explicit parameterless constructor            int main  int argc  const char   argv           const int howManyNames   3      name someName howManyNames       return 0        BTW  you need to include  to make the code compilable    An alternative way is to initialize your instances explicitly on declaration    name someName howManyNames        Ivan    The Terrible      Catherine    The Great

User · Answer

you just need to add a default constructor to your class to look like this   class name     public      string first      string last     name            name string a  string b       first   a      last   b

User · Answer

Your class   class name     public      string first      string last     name          Default constructor     name string a  string b       first   a      last   b           Has an explicit constructor that requires two string parameters   Classes with no constructor written explicitly get default constructors taking no parameters   Adding the explicit one stopped the compiler from generating that default constructor for you   So  if you wish to make an array of uninitialized objects  add a default constructor to your class so the compiler knows how to create them without providing those two string parameters - see the commented line above

User · Answer

To default-initialize an array of Ts  T must be default constructible  Normally the compiler gives you a default constructor for free  However  since you declared a constructor yourself  the compiler does not generate a default constructor   Your options    add a default constructor to name  if that makes sense  I don t think so  but I don t know the problem domain   initialize all the elements of the array upon declaration  you can do this because name is an aggregate      name someName 4         Arthur    Dent                               Ford    Prefect                               Tricia    McMillan                               Zaphod    Beeblebrox                             use a std  vector instead  and only add element when you have them constructed

[c++] How do I declare an array with a custom class?

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