Fixing initialization of structs for which constructors have been added

Question

A while ago I wrote code resembling this:

struct Thing
{
    int a,b;
};

struct Junk
{
    double x;
    Thing things[10];
};


Junk myjunk[] =
{
    {   23.4, {  {10,20}, {30,40} } },
    {   45.6, {  {55,66}, {77,88} } }
};

Originally Thing had no constructors - just a flat simple struct to hold a couple integers. myjunk[] is meant to be special hand-written data to feed into something. In real life it has dozens of lines not just two. This code compiled fine.

Then this morning I added constructors to Thing, useful elsewhere in the great scheme of things, for example, to call functions like blarf(Thing(123,456)); and create huge arrays on the heap with 'new'.

struct Thing
{
   int a,b;
   Thing();
   Thing(int _a, int _b);    
};

Now it doesn't compile. I get

Error 1 error C2552: 'Junk::things' : non-aggregates cannot be initialized with initializer list

Yes, class with constructors cannot be made this way. What is the quickest simplest fix to get this compiling?

I know C++11 allows some new ways to initialize variables, but it's not clear in this case what's best. I'm not creating a lone Thing, but an array, and that within another struct. 'Junk' will never have a constructor.

Note that there aren't any virtual methods anywhere, and never will be for Thing and Junk. I'm using Visual Studio 2010, and not sure if it's taking source to be C++11 or older C++.

See this for C++11: http://stackoverflow.com/questions/4178175/what-are-aggregates-and-pods-and-how-why-are-they-special/7189821#7189821 . And this: http://stackoverflow.com/questions/146452/what-are-pod-types-in-c for pre-11. — aruisdante, Jan 16 '15 at 18:18
It actually compiles fine under [clang](http://melpon.org/wandbox/permlink/5oUo0QH6JY7EIolr). But VC++ is different story of-course. — AnArrayOfFunctions, Jan 16 '15 at 19:11

ecatmur · Answer 1 · 2015-01-16T18:25:43.493

2

Change {10, 20} to Thing(10, 20) and so forth.

If you make the constructors constexpr conditional on language version then once you upgrade to a modern compiler you will be able to have the objects initialized at compile-time again:

#if __cplusplus >= 201103L
constexpr
#endif
Thing() : a(0), b(0) {}

#if __cplusplus >= 201103L
constexpr
#endif
Thing(int a, int b) : a(a), b(b) {}

edited Jan 16 '15 at 18:25

answered Jan 16 '15 at 18:20

ecatmur

152,476
27
293
366

score 1 · Answer 2 · answered Jan 16 '15 at 18:27

1

Your code is valid since C++11, so your compiler must not support that. What you can do is initialize the arrays like this:

Junk myjunk[] =
{
    {   23.4, {  Thing(10,20), Thing(30,40) } },
    {   45.6, {  Thing(55,66), Thing(77,88) } }
};

answered Jan 16 '15 at 18:27

juanchopanza

223,364
34
402
480

score 1 · Answer 3 · answered Jan 16 '15 at 19:01

You can add constructor for Junk with follows

Junk(double _x, initializer_list<Thing> tl) : x(_x){
        int i = 0;
        for (auto & t : tl) {
            things[i] = t;
            ++i;
            if (i >= 10) {
                break;
            }
        }
    }

Fixing initialization of structs for which constructors have been added

3 Answers3