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POPassing a constant matrix
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copied!<p>Referring to <a href="https://stackoverflow.com/questions/546860/passing-arrays-and-matrices-to-functions-as-pointers-and-pointers-to-pointers-in">this</a> question and especially the accepted <a href="https://stackoverflow.com/questions/546860/passing-arrays-and-matrices-to-functions-as-pointers-and-pointers-to-pointers-in/546982#546982">answer</a> of litb, I wonder why the gcc complain about this:</p> <pre><code>void func(const int (*ip)[3]) { printf("Value: %d\n", ip[1][1]); } int main() { int i[3][3] = { {0, 1, 2} , {3, 4, 5}, {6, 7, 8} }; func(i); return 0; } </code></pre> <p>If I eliminate the <code>const</code> the compiler keeps still. Did I something misunderstand? I wanted to be sure that <code>func</code> don't modify my array.</p> <p><b>EDIT:</b> The same thing happens if I define a data type for my matrix:</p> <pre><code>typedef int Array[3][3]; void func(const Array *p) { printf("Value: %d\n", (*p)[1][1]); } int main() { Array a = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8} }; func(&a); return 0; } </code></pre> <p>I accept, this kind of code isn't very C style, more like C++. In C++ indeed there would be no problem if I define Array as a class containing all the matrix behavior.</p> <pre><code>class Array {...}; </code></pre> <p>I suppose, I didn't understand very well the concept of arrays and arrays of arrays in C and passing them to functions. Any enlightenment?</p> <p>Thank you in advance.</p> <p><b>EDIT2:</b> Meanwhile I chewed a bit on this problem and it seems to converge to the following question: C/C++ implicitly converts a pointer to an <code>int</code> to a pointer to an <code>const int</code>. Thus the following works:</p> <pre><code>func(const int a[]) // aquivalent: func(const int *a) { ... } int main() { int b[10]; func(b); return 0; } </code></pre> <p>But C/C++ don't implicitly converts a pointer to an array of n <code>int</code>s to a pointer to an array of n <code>const int</code>s. Even though an array of n <code>int</code>s is implicitly converted to an array of n <code>const int</code>s. This level of indirection in the implicit conversion isn't supported. The following will be rejected (at least with a warning in C):</p> <pre><code>func(const int a[][n]) // aquivalent: func(const int (*a)[n]) { ... } int main() { int b[m][n]; func(b); return 0; } </code></pre> <p>It's similar to the problem that C++ doesn't implicitly convert a template for the type A into a template of type B even if A can be implicitly converted to B. The two templates are of completely different types.</p> <p>Is this the right answer?</p>

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