Passing arrays as reference arguments in function

Question

I am trying to pass two multidimensional arrays as reference arguments to a function in C++.

The prototype is written like this:

void func(char (&foo)[4][4], char (&bar)[4][4]);

My problem is that the second argument does not get passed as a reference argument, but the first one does. If I change their placement – so (&foo)[4][4] is second and vice versa – foo does not get passed as a reference but bar does.

However if I add a third argument, the second argument gets passed as reference.

How can I fix this without having to add another useless argument?

What do you mean by "it does not get passed as a reference"? — Oliver Charlesworth
– Oliver Charlesworth, Commented Dec 11, 2011 at 16:42
What else does it get passed as ? Copy ? No way in c++. C++ passes arrays as reference even if you think you're passing a copy. — ScarletAmaranth
– ScarletAmaranth, Commented Dec 11, 2011 at 16:44
Please post some actual code that demonstrates the problem (see sscce.org). — Oliver Charlesworth
– Oliver Charlesworth, Commented Dec 11, 2011 at 16:48
Post the whole related thing and show what you can't change. I am pretty sure that function is able to change it's callers memory. — ScarletAmaranth
– ScarletAmaranth, Commented Dec 11, 2011 at 16:51

Kerrek SB · Accepted Answer · 2011-12-11 16:58:47Z

3

Try to minimize the size of the proverbial foot to shot yourself into by using typedefs:

typedef char charray44[4][4];

void foo(charray44 & one, charray44 & two);

This should work as expected; and if not, you should be able to set up a simple test harness to figure out where (else) in the code you are making a mistake.

answered Dec 11, 2011 at 16:58

Kerrek SB

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Xeo · Accepted Answer · 2011-12-11 16:45:47Z

1

void foo(char (&one)[4][4], char (&two)[4][4]){
}

int main(){
    char one[4][4];
    char two[4][4];
    foo(one, two);
}

Compiles fine on MSVC, GCC and Clang. Your problem lies elsewhere.

answered Dec 11, 2011 at 16:45

Xeo

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111111 · Accepted Answer · 2011-12-11 16:55:25Z

1

As an addition to Xeos solution, a perhaps better way to solve this problem would be to encapsulate the array and pass the new object by reference.

class matrix{
     std::vector<char> values;
     size_t x_sz, y_sz;
public:
     matrix(size_t x, size_t y) : values(x*y), x_sx(x), y_sz(y) {}

     char& get_at(size_t x, size_t y) 
     { return values[y*x_sz+x]; }

     const char& get_at(size_t x, size_t y) const
     { return values[y*x_sz+x]; }
 };

then you can just pass to funcs like.

   void f(const matrix& a);

answered Dec 11, 2011 at 16:55

111111

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