Returning a pointer to array of fixed sized arrays C++

Question

I tried searching everywhere, but because it is such a perplexing question, I wasn't able to find what I was looking for. I am trying to create function/method but I don't know how to specify its return type, which should be:

double(*)[3]

I want to be able to use a query like this one:

double R[3][3];
query ( &output, R );

but instead of R[3][3], I have a vector std::vector<double> R_vect (9); and I do this:

query ( &output, reinterpret_cast<double(*)[3]> (R_vect.data()) );

which is a mess, so I wanted to implement a function to make it readable, say:

ReturnType Cast ( const std::vector<double>& R_vect ) {
  return reinterpret_cast<double(*)[3]> (R_vect.data());
}

but I can't specify the return type. I used typedef, and it works:

typedef double DesiredCast[3];
DesiredCast* Cast ( ... ) { ... }

but I am still curious how to do it without typedefs.

I'd return an instance of a class instead of returning a raw pointer at all. — Jerry Coffin
– Jerry Coffin, Commented Mar 20, 2015 at 23:39
Always use typedefs when using pointers or references to arrays or functions. It just gets insane so fast if you don't. — Mooing Duck
– Mooing Duck, Commented Mar 20, 2015 at 23:46
Is there a recommendation on how to name those typedefs to be clear what their purpose is? — tepsijash
– tepsijash, Commented Mar 20, 2015 at 23:58

score 4 · Accepted Answer · 2015-03-20 23:55:20Z

4

You should always typedef complicated return types like these, rather than require the reader to untangle them. (or redesign so you don't have complicated types!)

But you can just follow the pattern. To declare a variable of this type you would do

double (*var)[3];

and to make it a function you just put the usual decoration in the usual place next to the name, despite horrible it seems. e.g. with an int argument named z:

double (*func(int z))[3]
{
    // ...
}

Incidentally, cdecl will do that for you, once you learn its language.

edited Mar 20, 2015 at 23:55

answered Mar 20, 2015 at 23:44

user1084944

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1 Comment

tepsijash Over a year ago

Thanks. That was what I was looking for. Seeing the syntax, I believe I will stick with the typedef :D Also, for the sake of completeness, here is the code for a method (which is also confusing): double (*Class::func(void))[3]

jschultz410 · Accepted Answer · 2015-03-20 23:44:42Z

0

It's pretty bizarre syntax:

double (*foo(void))[3]
{
  static double f[3][3];

  return f;
}

answered Mar 20, 2015 at 23:44

jschultz410

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