Assigning class variables via named parameters in constructor call in C++

Question

I am coming from C# background. There I could do something like this:

Callback callback = new Callback
    {
        src = src,
        callbackPtr = callbackPtr
    };

assuming there is only an empty constructor:

Callback(){}

and both variables are public.

Is there something like that in C++ or do I have to create parameterized constructor here?

Could you explain what it means in C#, for those of us who don't know it? — Kerrek SB
– Kerrek SB, Commented Sep 6, 2013 at 22:07
BTW That's a fairly new feature in C#, before you had to write Callback callback = new Callback(); callback.src = src; callback.callbackPtr = callbackPtr; ... and that idea works in C++ as well. — Ben Voigt
– Ben Voigt, Commented Sep 6, 2013 at 22:15
@KerrekSB It's a default constructor with an intializer for the named members src and callbackPtr. This would be really dangerous in C++. — marko
– marko, Commented Sep 6, 2013 at 22:53

nneonneo · Accepted Answer · 2013-09-06 22:13:32Z

4

If Callback is a plain-old-data (POD) type, you can do this:

Callback callback = {src, callbackPtr};

which uses the structure initialization syntax. It assumes that src and callbackPtr are listed in that order in the class definition.

This works in both C++03 and C++11. In C++11, non-POD classes can use this syntax by defining a constructor that takes a std::initializer_list.

answered Sep 6, 2013 at 22:13

nneonneo

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1 Comment

juanchopanza Over a year ago

In C++11, it can also work for a class with a two-parameter constructor.

Janick Bernet · Accepted Answer · 2013-09-06 22:20:32Z

0

You can do:

Callback* callback = new Callback
    {
        src,
        callbackPtr
    };

Assuming Callback has fields with the same data types in that order and - big caveat - the fields have to be public.

edited Sep 6, 2013 at 22:20

answered Sep 6, 2013 at 22:04

Janick Bernet

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9 Comments

Chebz Over a year ago

I am receiving "No matching constructor for initialization" compile error. Even though I defined empty ctor: Callback() {}

nneonneo Over a year ago

This calls a two-argument constructor using the uniform constructor syntax. It is not the same as the C# construct.

Chebz Over a year ago

but then what's the benefit using that over new Callback(src, callbackPtr); ? I was trying to avoid using parameterized ctor.

Wayne McGee Over a year ago

There is no benefit aside from uniform notation, unless you are dealing with template code.

Ben Voigt Over a year ago

This will work if Callback is an aggregate. class vs struct doesn't matter. Having public member variables and no constructor does matter.

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