Can I make a constructor in C++ of different data types?

Question

I have a class which has multiple data members of different data types. I was wondering if I can create and initialize an object using a constructor.

class test
{
     public:
            int a;
            string b;
            float c;

      test (int x, string y, float z);

};


int main()
{

    test t1(10,"Hello",2.5f); //this line doesn't work

}

_"I was wondering if I can create and initialize an object using a constructor."_ - that's basically the sole reason constructors exist ;) Please add some more details to your question to clarify _what exactly_ is not working. You only declared the constructor, but never defined it in the code shown. You also didn't include ``. — Lukas-T, Jun 14 '21 at 11:54
Yes you can do that. Please show a more complete example and the exact error you are getting. — Cory Kramer, Jun 14 '21 at 11:55
oh damn, my bad i completely forgot to define the constructor, i keep forgetting to define them and think that the values will automatically get assigned, i should have assigned the values inside the constructor, thanks @churill stupid mistake on my part — Free-Man, Jun 14 '21 at 12:00

Bathsheba · Answer 1 · 2021-06-14T12:25:10.533

1

You need to implement the constructor test (int x, std::string y, float z); Don't forget to #include <string> at the top of your source file.

Although you could use the notation

test t1{10, "Hello", 2.5f};

which will cause the arguments to be assigned to the class members in the order they are written; although you will have to drop the constructor completely.

edited Jun 14 '21 at 12:25

answered Jun 14 '21 at 11:56

Bathsheba

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score 0 · Answer 2 · answered Jun 14 '21 at 11:57

0

In your example you have declared the constructor, but never defined it.

class test
{
public:
    int a;
    std::string b;
    float c;

    test(int x, std::string y, float z) : a(x), b(y), c(z) {}
};


int main()
{
    test t1(10, "Hello", 2.5f);
}

answered Jun 14 '21 at 11:57

Cory Kramer

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Victor · Accepted Answer · 2021-06-14T14:52:31.950

0

The fact that it doesn't work is not really what we do on StackOverflow. You should provide an error message.

With that said, I suppose you are getting an error from the linker because your constructor does not include a definition, but only a declaration.

Here's a working example: https://godbolt.org/z/r89cf86s3

#include <string>

class test
{
public:
    int a;
    std::string b;
    float c;

    test (int x, std::string y, float z) : a{x}, b{y}, c{z}
    {}
};


int main()
{
    test t1(10, "Hello", 2.5f);
}

NOTE Passing std::string by value to the constructor can lead to temporary objects being created, so to keep the workings of this code and improve it, you could use a const-reference, i.e. const std::string& as parameter of the constructor.

edited Jun 14 '21 at 14:52

answered Jun 14 '21 at 11:57

Victor

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Just got it thanks, also i never knew that we could assign the values like a{x}, b{y} and so forth, so thanks again – Free-Man Jun 14 '21 at 12:04
@Free-Man [I hate to be that guy, but...](https://www.urbandictionary.com/define.php?term=I%20hate%20to%20be%20that%20guy%2C%20but...) this will cause unnecessary temporaries. – Aykhan Hagverdili Jun 14 '21 at 12:32
@AyxanHaqverdili "Often secretly means, "I love to be that guy", " :P – 463035818_is_not_an_ai Jun 14 '21 at 12:38
@AyxanHaqverdili because of the `std::string` not being passed as const reference you mean? Is there any other? – Victor Jun 14 '21 at 14:45
@Victor yes, that one. You could also take-by-value-and-move. That'd also avoid the extra copy. – Aykhan Hagverdili Jun 14 '21 at 14:50
I saw it straight from the beginning, but wanted to keep the spirit of the original code. – Victor Jun 14 '21 at 14:51
@Victor nothing wrong with your answer. Just wanted to let the OP know that. – Aykhan Hagverdili Jun 14 '21 at 14:52

score -1 · Answer 4 · edited Jun 14 '21 at 12:05

-1

I forgot to define the constructor :(

it should've been like this:-

class test
{
     public:
            int a;
            string b;
            float c;

      test (int x, string y, float z)
    {
        a=x;
        b=y;
        c=z;

     }

};

int main() {

test t1(10,"Hello",2.5f); //this line doesn't work

}

edited Jun 14 '21 at 12:05

kometen

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answered Jun 14 '21 at 12:02

Free-Man

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We've all answered that, yes :) – Victor Jun 14 '21 at 12:03
1

@Bathsheba You [can](https://gcc.godbolt.org/z/4vjxboedn). Objects are not const within their own constructors/destructors. – Aykhan Hagverdili Jun 14 '21 at 12:36
@AyxanHaqverdili, sorry, senior moment. I'll put the real reason up. – Bathsheba Jun 14 '21 at 12:38
1

Using base member initialisation is preferred in constructors in order that `const` members can be set. – Bathsheba Jun 14 '21 at 12:39
when you write "should be like this" you put the bar a little higher and not using the member initializer list doesn't pass for me. Read here: https://stackoverflow.com/questions/926752/why-should-i-prefer-to-use-member-initialization-lists – 463035818_is_not_an_ai Jun 14 '21 at 12:40

Can I make a constructor in C++ of different data types?

4 Answers4