how to initialize array of structures having const variables

Question

I have an array of struct having const variables; I am initialize by using constructor initializer list, but if I make an array of that struct how to initialise all the elements of array of that structure. if the struct contains no const var, then it is easily done

if the struct contains no const var, then it is easily done

#include<iostream>
struct s_Nconst{
    int num;
    int x1;
    // s_Nconst(int);
};
int main(){
    s_Nconst sN2;
    sN2.num=5;
    std::cout<<"initial values of num\t"<<sN2.num<<std::endl;   

    //making array of s_Nconst struct
    s_Nconst sN3[sN2.num];

    for(int i=sN2.num-1; i>-1; i--,++sN2.num){
        sN3[i].x1= sN2.num;
        std::cout<<"sn3["<<i+1<<"]\t"<<sN3[i].x1<<std::endl;
    }

    return 0;
}

but if suppose the struct comprises of const var, then I am able to make single object of the struct using the constructor initializer list, but if I am going to make array of the struct it is giving error

#include<iostream>

struct s_const{
    const int x1,y1,z1;
    static int num;
    s_const(int,int,int);
};
s_const::s_const(int x,int y, int z)
        :x1(x),y1(y),z1(z) {

    std::cout<<"initial values of x1,y1,z1\t"<<this->x1<<" "<<this->y1<<" "<<this->z1<<std::endl;   
}


/*struct s_Nconst{
    int num;
    int x1;
    // s_Nconst(int);
};
 s_Nconst::s_Nconst(int x)
        :num(x){

    std::cout<<"initial values of num\t"<<this->num<<std::endl;   
} */


int main(){

    int a,b,c;
    a=1;b=2;c=3;
    s_const s1(a,b,c);

    //array of s1 is giving error
    s_const sc[3];




    //s_Nconst s2(++c);
    //s_Nconst sN2;
    //sN2.num=5;
    //std::cout<<"initial values of num\t"<<sN2.num<<std::endl;   

    //making array of s_Nconst struct
    //s_Nconst sN3[sN2.num];

    //for(int i=sN2.num-1; i>-1; i--,++sN2.num){
    //    sN3[i].x1= sN2.num;
        std::cout<<"sn3["<<i+1<<"]\t"<<sN3[i].x1<<std::endl;
    //}

    return 0;
}

is there any way to make array of structs having const variables and how to initialize them?

sN3[i].x1= sN2.num; like you are doing in non-const example, isn't initialization. It's assignment. s_const sc[3] = {s_const (1, 2, 3), s_const (4, 5, 6), s_const (7, 8, 9)}; would be initialization. — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 26, 2019 at 14:52
It's hard to tell what you actually want to do because there is a lot broken code commented out (leaving some of the remaining code broken). But one option would be s_const sc[3]{ {1,1,1}, {1,1,1}, {1,1,1} };. — Max Langhof
– Max Langhof, Commented Sep 26, 2019 at 14:52
s_Nconst sN3[sN2.num]; is invalid and use VLA extension, prefer std::vector. — Jarod42
– Jarod42, Commented Sep 26, 2019 at 14:52
@DetectivePickachu a constructor can initialize also const members. "Whatever value they have when you declare them is the value they will have for the entire execution." - not true — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 26, 2019 at 14:53
@DetectivePikachu sorry, but your statements are completely wrong. Constructors role is exactly to set the values of all members, including the const-qualified ones. — SergeyA
– SergeyA, Commented Sep 26, 2019 at 14:56

SergeyA · Accepted Answer · 2019-09-26 14:53:03Z

4

You can create an array of const objects, but they have to be initialized. For example, for your structs it should be:

s_const sc[3] = {{1, 2, 3}, {3, 4, 5}, {6, 7, 8}};

On a side note, you might prefer std::array to built-in arrays.

answered Sep 26, 2019 at 14:53

SergeyA

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Ayjay Over a year ago

Just s/might/should

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