Cpp/C++ Assign Default value to default constructor

Question

This seems like a really easy question but I can't find a working solution(maybe it is the rest of the code too). So basically how do you assign a value to an object created with the default constructor, when the custom constructor has that variable as a parameter? (hopefully this is understandable)

Maybe clearer: The code below only works if I write foo ex2(2) instead of foo ex2() inside the main function. So how do I assign a default value to x if the object is created with the default constructor

class foo {

public:
    int y;
    int x;
    static int counter;

    foo()
    {
        y = counter;
        counter++;
        x = 1;
    };

    foo(int xi) 
    {
        y = counter;
        counter++;
        x = xi;
    };

};

int foo::counter = 0;

int main()
{

    foo ex1(1);
    foo ex2();

    std::cout << ex1.y << "\t" << ex1.x << "\n";
    std::cout << ex2.y << "\t" << ex2.x << "\n";
    std::cin.get();
};

You already are assigning a value with x = 1; - I'm not sure what else you want to do — UnholySheep
– UnholySheep, Commented Nov 26, 2021 at 20:43
You should turn on all your compilers warnings: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse] -> What is the purpose of the Most Vexing Parse? — Lukas-T
– Lukas-T, Commented Nov 26, 2021 at 20:44
@UnholySheep my bad the problem was the wrong object declaration of ex2 so it didn't work. Ty tho :) — Finn
– Finn, Commented Nov 26, 2021 at 20:50

Vlad from Moscow · Accepted Answer · 2021-11-26 20:53:46Z

1

This record

foo ex2();

is not an object declaration of the class foo.

It is a function declaration that has the return type foo and no parameters.

You need to write

foo ex2;

or

foo ( ex2 );

or

foo ex2 {};

or

foo ex2 = {};

edited Nov 26, 2021 at 20:53

answered Nov 26, 2021 at 20:46

Vlad from Moscow

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Comments

lakeweb · Accepted Answer · 2021-11-26 20:51:52Z

0

As said above. Then to assign a default value:

class foo {
public:
    int y;
    int x;
    static int counter;
    foo(int xi = 1) : x(xi)
    {
        y = counter;
        counter++;
    };
};
int foo::counter = 0;

int main()
{
    foo ex1(3);
    foo ex2;

    std::cout << ex1.y << "\t" << ex1.x << "\n";
    std::cout << ex2.y << "\t" << ex2.x << "\n";
};

edited Nov 26, 2021 at 20:51

answered Nov 26, 2021 at 20:48

lakeweb

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3 Comments

Lukas-T Over a year ago

Actually, you made another, mor important change to the original code than providing a default value for xi.

lakeweb Over a year ago

Hi @churill, do you mean the foo ex2; part?

Lukas-T Over a year ago

Excatly :) foo ex2(); (like in the original) would also fail in your version.

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