CODEWITHSUNDEEP
c++
chintan s
chintan s

Reputation: 6488

How to initialise a member array of class in the constructor?

I am trying to do the following:

class sig
{
public:

int p_list[4];
}

sig :: sig()
{
p_list[4] = {A, B, C, D};
}

I get an error

missing expression in the constructor.

So how do I initilalise an array?

Upvotes: 9

Views: 44187

Answers (6)

Mayur Lokare
Mayur Lokare

Reputation: 31

You can initialise the array members like this using c++11 compiler using -std=c++11 or -std=gnu++11 option 

struct student {
        private :
                int marks[5];
        public :
                char name[30];
                int rollno;
                student(int arr[], const char *name, int rno):marks{arr[0], arr[1], arr[2], arr[3], arr[4]}{
                        strcpy(this->name, name);
                        this->rollno = rno;
                }
                void printInfo() {
                        cout <<"Name : "<<this->name<<endl;
                        cout <<"Roll No : "<<this->rollno<<endl;
                        for(int i=0; i< 5; i++ ) {
                                cout <<"marks : "<<marks[i]<<endl;
                        }
                }
};
int main(int argc, char *argv[]) {
        int arr[] = {40,50,55,60,46};
        //this dynamic array passing is possible in c++11 so use option -std=c++11
        struct student s1(new int[5]{40, 50, 55, 60, 46}, "Mayur", 56);
        //can't access the private variable
        //cout <<"Mark1 : "<<s1.marks[0]<<endl;
        s1.printInfo();`enter code here`
}

Upvotes: 1

PaperBirdMaster
PaperBirdMaster

Reputation: 13320

If your current compiler doesn't yet support C++11, you can initialize the vector contents using standard algorithms and functors:

class sig
{
public:
    sig()
    {
        struct Functor
        {
            Functor() : value(0) {};
            int operator ()() { return value++; };
            int value;
        };
        std::generate(p_list, p_list + 4, Functor());
    }

    int p_list[4];
};

Previous snippet example here.

Yes, is kind of ugly (at least, it looks ugly for me) and doesn't do the work at compile time; but it does the work that you need in the constructor.

If you need some other kind of initialization (initialization with even/odd numbers, initialization with random values, start with anoter value, etc...) you only need to change the Functor, and this is the only advantage of this ugly approach.

Upvotes: 2

Konrad Rudolph
Konrad Rudolph

Reputation: 545518

So how do I initilalise an array?

Using the normal initialiser list syntax:

sig::sig() : p_list{1, 2, 3, 4}
{ }

Note, this only works in C++11. Before that, you need to use a boost::array an initialise it inside a function.

Upvotes: 8

Some programmer dude
Some programmer dude

Reputation: 409136

If your compiler doesn't support C++11 initialization, then you have to assign each field separatly:

p_list[0] = A;
p_list[1] = B;
p_list[2] = C;
p_list[3] = D;

Upvotes: 5

Kerrek SB
Kerrek SB

Reputation: 476950

In C++11 only:

class sig
{
    int p_list[4];
    sig() : p_list { 1, 2, 3, 4 }   {   }
};

Pre-11 it was not possible to initialize arrays other than automatic and static ones at block scope or static ones at namespace scope.

Upvotes: 19

okay_google
okay_google

Reputation: 349

p_list[4] = {'A', 'B', 'C', 'D'};

Upvotes: -3

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