Do templates need overloaded constructors?

Question

I just finished watching some videos of template and I think I am missing some concepts. Why doesn't the constructor get called, or why the object is not created when the constructor is not overloaded with a desired data-type? Since I am writing the <int> doesn't the compiler know I am going to be dealing with an int?

template <class T>
class Generic {
    T var;
public:
    Generic(){cout << "ctor called " << endl;}
    //Generic (T v) {var = v;}
};


int main () {

    Generic<int> generic1();

}

Can't I create an object like this and then modify the value of T var through a setter? Why should I need an overloaded constructor e.g. Generic<int> generic1(9); ?

Lookup "the most vexing parse". You need to use `Generic generic1;` — R Sahu, Sep 18 '17 at 15:30
@vsoftco I don't think that dupe is right. I believe the OP asking why they should provide a constructor that takes a value when they could just write a setter. — NathanOliver, Sep 18 '17 at 15:34
@NathanOliver You're right, reopened. I just saw it on the first line of main :) But it's a combination of both. — vsoftco, Sep 18 '17 at 15:36
It was my mistake, I had extra '()' while creating an object with no params. — Andy, Sep 18 '17 at 15:40

songyuanyao · Accepted Answer · 2017-09-18T16:01:56.853

8

This is a Most vexing parse issue.

Of course you can initialize the object via the default constructor, and modify the value via a setter later, the problem here is that you're not defining a variable. Generic<int> generic1(); is a declaration of function, which is named generic1, takes no arguments and returns Generic<int>.

What you want is

Generic<int> generic1;

or

Generic<int> generic1{}; // since C++11

edited Sep 18 '17 at 16:01

answered Sep 18 '17 at 15:31

songyuanyao

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Yes, you are correct. My mistake I didn't see that. – Andy Sep 18 '17 at 15:34

Do templates need overloaded constructors?

1 Answers1