Declaring a variable size array/vector inside a C++ class

Question

I want to declare a variable size (not really variable size) array/vector inside a C++ class and I am encountering weird errors. I have tried to find a solution to this but cannot resolve the error. C++ gods, bless me.

class threeStack{
    private:
        int numberOfStacks = 3;
        int sizeOfEachStack = 2;
        int kc = 10;
        vector <int> a(numberOfStacks * sizeOfEachStack);
        vector <int> tops(numberOfStacks);
        };

ERRORS

         vector <int> a(numberOfStacks * sizeOfEachStack);
                        ^~~~~~~~~~~~~~
main.cpp:10:27: error: ‘numberOfStacks’ is not a type
         vector <int> tops(numberOfStacks);
                           ^~~~~~~~~~~~~~

Why do you think you can do `vector a(n);`? Which [book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) says you can? — Evg, Jul 08 '21 at 12:03
@Evg if I have defined n before `vector a(n);`, I can definitely use that. — Anant, Jul 08 '21 at 12:06
@Anant Inside a function body, not inside a class definition. See [here](https://stackoverflow.com/questions/24836526/why-c11-in-class-initializer-cannot-use-parentheses). — Evg, Jul 08 '21 at 12:08

dbush · Answer 1 · 2021-07-08T12:27:01.343

You don't specify constructor arguments to class members when they are declared. That's done in the contructor of the class itself.

class threeStack{
    private:
        int numberOfStacks;
        int sizeOfEachStack;
        int kc;
        vector <int> a;
        vector <int> tops;

    public:
        threeStack() : numberOfStacks(3), sizeOfEachStack(2), 
                       kc(10), a(3 * 2), tops(3) {
        }

        threeStack(int numberOfStacks, int sizeOfEachStack) : 
            numberOfStacks(numberOfStacks),
            sizeOfEachStack(sizeOfEachStack),
            kc(10), 
            a(numberOfStacks * sizeOfEachStack), 
            tops(sizeOfEachStack) 
        {
        }
};

Also, default initialization of data members is a feature of C++11 and later, so if you're compiling with an older version of the standard you'll need to specify those in the constructor as well.

"You don't specify constructor arguments to class members when they are declared, nor can you specify default values." Of course you can. It is called default member initializer — phön, Jul 08 '21 at 12:23

eerorika · Answer 2 · 2021-07-08T12:20:15.357

0

This is declaration of a function:

vector <int> tops(numberOfStacks);
|            |   ||
|            |   |type of first parameter
|            |   parameter list
|            name of the function
return type

You cannot use this syntax for default member initialiser.

edited Jul 08 '21 at 12:20

answered Jul 08 '21 at 12:05

eerorika

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can you recommend me something which I can read to learn this stuff? atleast the basics, I am currently reading "accelerated c++" though. – Anant Jul 08 '21 at 12:09
@Anant There's a list of books [here](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). I cannot vouch for any particular book myself. My only C++ book is some old edition of Bjarne's The C++ Programming Language. – eerorika Jul 08 '21 at 12:13
2

"Also, you cannot refer to other members in a default member initialiser." I think you can? http://coliru.stacked-crooked.com/a/2824f8cf7a4ff0c6 – phön Jul 08 '21 at 12:18
1

@phön I guess I just assumed that ¯\\_(ツ)_/¯ – eerorika Jul 08 '21 at 12:20
@eerorika ¯\\_(ツ)_/¯ – phön Jul 08 '21 at 12:21

Declaring a variable size array/vector inside a C++ class

2 Answers2