C++ Initialize object member of a struct that is allocated using the "new" keyword

Question

This may be a complicated question but I assume it has a simple answer.

So I have a struct that contains custom object types:

// header1.h

struct MyStruct
{
    MyClass myObject1;
    MyClass myObject2;
};

And I have a global pointer to a MyStruct. I am going to allocate and instantiate the struct using the new keyword. When I do so, I want to construct each myObject by name in an initializer list, as shown below.

// codeFile1.cpp

MyStruct *myStructPtr;

int main()
{
    myStructPtr = new MyStruct
    {                                     //syntax error: missing ';'
        .myObject1 = MyClass(arg1, arg2),
        .myObject2 = MyClass(arg1, arg2)
    };
}

The code snippet above has syntax errors, but it demonstrates what I want to do. Note that MyClass does not have a default constructor, so it must be given an argument list.

Thanks!

EDIT - CONCLUSION

As pointed out, my initialization list is C-style, which fails to work. Unfortunately the C++11 initializer-list does not meet my compiler requirements, and the option of throwing all arguments into a constructor is not practical.

So I took an alternative solution and changed the pointer structure to the following:

// header1.h

struct MyStruct
{
    MyClass *myObjectPtr1;
    MyClass *myObjectPtr2;
};

// codeFile1.cpp

MyStruct myStruct;

int main()
{
    myStructPtr.myObjectPtr1 = new MyClass(arg1, arg2);
    myStructPtr.myObjectPtr2 = new MyClass(arg1, arg2);
}

Thanks all!

Is `MyStruct` an aggregate? If not, you need to provide a constructor. — Pradhan, Feb 14 '15 at 00:01
Also, what you have shown here is `C`'s [tagged struct initialization syntax](http://stackoverflow.com/questions/3016107/what-is-tagged-structure-initialization-syntax). C++ doesn't support it. — Pradhan, Feb 14 '15 at 00:03
@Pradhan I believe so. It is parallel to the the code shown here, the only significant difference is the number of members. — Aaron Campbell, Feb 14 '15 at 00:14
@LightnessRacesinOrbit That happened a while ago, yes. Forgot to edit it from my profile. — Aaron Campbell, Feb 14 '15 at 00:22

score 3 · Answer 1 · answered Feb 14 '15 at 00:05

3

If MyStruct is an aggregate, you could use aggregate initialization to avoid defining a constructor:

myStructPtr = new MyStruct {{arg1, arg2}, {arg1, arg2}};

If not, please provide a constructor taking the appropriate arguments and call

myStructPtr = new MyStruct{arg1, arg2};

answered Feb 14 '15 at 00:05

Pradhan

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Using the aggregate initialization doesn't get rid of the "missing ';'" syntax error. Also seing a "MyStruct: no appropriate default constructor available" on the same line. – Aaron Campbell Feb 14 '15 at 00:18
This is _not_ aggregate initialization, guys. – Lightness Races in Orbit Feb 14 '15 at 00:25
@LightnessRacesinOrbit I am not sure what I am missing. From N3797, `8.5.4 [dcl.init.list]`, the first point says that the above initializer expression for the `new` is *list initialization*. The third point in the same section says `If T is an aggregate, aggregate initialization is performed`. And my answer opens with the assumption that `MyStruct` is an aggregate. – Pradhan Feb 14 '15 at 01:09
`[C++11: 8.5.1/1]:` "An _aggregate_ is an array or a class (Clause 9) with no user-provided constructors (12.1), no _brace-or-equal-initializers_ for non-static data members (9.2), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3)." – Lightness Races in Orbit Feb 14 '15 at 01:13
Fair enough, though: _if_ `MyStruct` were an aggregate then it would be aggregate initialisation (TIL) so, although misleading, your answer is not strictly wrong the way it is worded. Just testing! ;) – Lightness Races in Orbit Feb 14 '15 at 01:14
@LightnessRacesinOrbit I am offended! My answer has an if clause and an else clause :) – Pradhan Feb 14 '15 at 01:32

score 2 · Accepted Answer · answered Feb 14 '15 at 00:05

2

You're trying to use C's designated initializers, which are not part of C++. If you have a C++11 compiler, you can simply do the following:

MyStruct *myStructPtr = new MyStruct
{
    MyClass(arg1, arg2),
    MyClass(arg1, arg2)
};

Live demo

answered Feb 14 '15 at 00:05

Praetorian

106,671
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Wow! I didn't know you could do this in general. Which list initialization clause covers this? when `MyStruct` isn't an aggregate? Running through the conditions [here](http://en.cppreference.com/w/cpp/language/list_initialization), I am unable to understand why this is allowed. – Pradhan Feb 14 '15 at 00:08
What makes this possible in C++11 but not earlier? – Aaron Campbell Feb 14 '15 at 00:09
@Pradhan It's the same as your answer, in fact, I like your answer better because you avoided mentioning `MyClass` – Praetorian Feb 14 '15 at 00:11
2

@aaronncfca It requires list initialization which is part of C++11 – Praetorian Feb 14 '15 at 00:11

Mats Petersson · Answer 3 · 2015-02-14T00:16:28.120

1

You will, somehow, need to chain your constructions together. Assuming you actually want the same arguments:

struct MyStruct
{
    MyStruct(int arg1, int arg2) : myObject(arg1, arg2), myObejct2(arg1, arg2) {}
    MyClass myObject1;
    MyClass myObject2;
};

MyStruct *s = new MyStruct(34, 42);

Or something like this:

struct MyStruct
{
    MyStruct(const MyClass& a1, const MyClass& a2) : myObject(a1), myObejct2(a2) {}
 .... 
};

MyStruct *s = new MyStruct(MyClass(1,2), MyClass(3,4));

or in C++11 initializer lists:

MyStruct *s = new MyStruct({1,2}, {3,4});

Many other similar solutions are available.

edited Feb 14 '15 at 00:16

answered Feb 14 '15 at 00:07

Mats Petersson

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I have about 50 members in the real MyStruct each with several arguments in the constructor, so this is not viable for me. – Aaron Campbell Feb 14 '15 at 00:10
wouldn't it also work doing: ... `MyStruct *s = new MyStruct({1,2}, {3,4});` ? – Juarrow Feb 14 '15 at 00:10
@aaronncfca: Maybe you could post the REAL code then, not something you just made up to post here - but that sounds like you probably don't want to initialize with an initializer list in that way anyway... – Mats Petersson Feb 14 '15 at 00:15
@aaron 50 members?! Are you sure your classes are not _wildly_ too large? – Lightness Races in Orbit Feb 14 '15 at 00:15
@LightnessRacesinOrbit It's a list of objects that I wanted to put in a struct to keep better manageable. – Aaron Campbell Feb 14 '15 at 00:20
@aaron It's an extremely long list. Consider rethinking it. – Lightness Races in Orbit Feb 14 '15 at 00:22
@LightnessRacesinOrbit How else would you suggest storing such a list of objects? They need to be globally accessible and dynamically allocated. – Aaron Campbell Feb 14 '15 at 00:24
@aaronncfca: There are like a million ways you could do it. An answer to that question could fill a book. Speaking books which one are you using? – Lightness Races in Orbit Feb 14 '15 at 00:25
Give REAL code, and we MAY be able to offer more useful advice. But it generally seems wrong to have 50 objects in a single struct. – Mats Petersson Feb 14 '15 at 00:27
Would it be superior to have all of them in global scope? The purpose in the struct is to minimize the number of global variables, which I feel is good practice (possibly for JavaScript reasons). – Aaron Campbell Feb 14 '15 at 00:49
Really can't answer that without understanding what you are doing. 50 odd things that are global seems wrong either way. – Mats Petersson Feb 14 '15 at 10:29

C++ Initialize object member of a struct that is allocated using the "new" keyword

3 Answers3