Declare a variable of a class without creating an instance of it

Question

I'm learning C++.

I want to declare a variable without creating an instance of it.

MyClass variable;

// More code

int main(int argc, char **argv)
{
   // More code
   variable = MyClass(0);

   // More code
}

If I do that, and MyClass only has a constructor declared MyClass::MyClass(int value) it fails.

I need to make it global because I'm going to use it on a CallBack function and I can pass that variable as a parameter.

And also, I don't want to create an instance of the class when I declare the variable and then, another instance when I use the constructor. I think I'm wasting resources and CPU time.

Is it possible to declare a variable without instance it?

Answer 1

Use a pointer to delay instantiation. Prefer smart pointers to raw pointers so you don't have to worry about manual memory management.

#include <memory>

std::shared_ptr<MyClass> variable;

// More code

int main(int argc, char **argv)
{
   // More code
   variable = std::make_shared<MyClass>(0);

   // More code
}

Whether you use std::unique_ptr or std::shared_ptr depends on what you plan to do with the pointer.

Answer 2

You can use a std::optional for this:

#include <optional>

std::optional<MyClass> variable; // not initialized

int main() {
    if (variable) {
        // enters this if only when initialized
    }

    variable = MyClass(0);

    // Access the contained value:
    MyClass value = *variable;
    int member = variable->member;
}

The std::optional type is available in C++17. If your project is constrained to an older version, you can always use a polyfill

Answer 3

I am not sure, but it seems like you have created a constructor like :

MyClass(int v) : value(v) {}

and not written the default constructor, which may look like :

MyClass() {}

Because of this, the compiler cannot find the constructor to instantiate a MyClass object when you write:

MyClass variable;

In short, you have to explicitly write the default constructor when you have written other constructors but also want to use the default one.

And your question is a bit vague, when you declare a variable of a class like above, you are indeed creating an instance of it. Even when doing

variable = MyClass(0);

you are creating a temporary object of MyClass and then using the assignment operator to assign it to the variable object.

Answer 4

You could just initialise the variable with some temporary value with the existing constructor when you declare it. Ideally with a value you could easily identify when debugging.

MyClass variable(-1);

Although then it would probably make more sense to just add a default constructor that does that.