Passing 2D array by reference
Author
What is the syntax for passing a 2D array (static) by reference?
I tried the following but it seems to be passing by value.
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int param[][2]) {}
foo(a);
And
void foo(int ¶m[][2]) {}
doesn't work...
#include <stdio.h>
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int param[][2]) {
param[0][0] = 5;
}
int main(){
foo(a);
printf("%d\n", a[0][0]);
}
It should print 5, so it's definitely not passing by value. In C static arrays are implemented using pointers, so they are passed by pointer.
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int param[][2]) {
param[0][0] = 5;
}
int main(){
foo(a);
printf("%d\n", a[0][0]);
}
It should print 5, so it's definitely not passing by value. In C static arrays are implemented using pointers, so they are passed by pointer.
Author
Quote:Original post by deathkrush
#include <stdio.h>
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int param[][2]) {
param[0][0] = 5;
}
int main(){
foo(a);
printf("%d\n", a[0][0]);
}
It should print 5, so it's definitely not passing by value. In C static arrays are implemented using pointers, so they are passed by pointer.
Oh... You're right. I have no idea why it didn't work for me.
But this brings another question:
How do I pass it by value then?
I'm pretty much a novice, but I would suggest to pass by value:
foo(&a)
Again, i'm a novice. However, it seems to me that you want to pass the entire array to the function. If that is the case, I would suggest a wrapper. If you can't do a wrapper, try this:
#include <stdio.h>
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int *param[][2]) {
param[0][0] = 5;
}
int main(){
foo(&a);
printf("%d\n", a[0][0]);
}
Let me know how it turns out ... I haven't actually tried the above method, but it might actually work.
foo(&a)
Again, i'm a novice. However, it seems to me that you want to pass the entire array to the function. If that is the case, I would suggest a wrapper. If you can't do a wrapper, try this:
#include <stdio.h>
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int *param[][2]) {
param[0][0] = 5;
}
int main(){
foo(&a);
printf("%d\n", a[0][0]);
}
Let me know how it turns out ... I haven't actually tried the above method, but it might actually work.
Author
Quote:Original post by argonaut
I'm pretty much a novice, but I would suggest to pass by value:
foo(&a)
Again, i'm a novice. However, it seems to me that you want to pass the entire array to the function. If that is the case, I would suggest a wrapper. If you can't do a wrapper, try this:
#include <stdio.h>
int a[][2] = { { 1, 2 }, { 3, 4 } };
void foo(int *param[][2]) {
param[0][0] = 5;
}
int main(){
foo(&a);
printf("%d\n", a[0][0]);
}
Let me know how it turns out ... I haven't actually tried the above method, but it might actually work.
Ok, you're confusing me. Why would foo(&a) be passing by value? It seems more like passing "the reference of the pointer to the array."
I haven't had the chance to test it yet, but it just look kinda odd to me.
You cannot pass c-style arrays by value. You can either make them const to prevent accidental manipulation, or make them in a small structure and pass that by value.
Or just use std::vector for all you array-like needs.
If you have boost, try boost::array.
Or just use std::vector for all you array-like needs.
If you have boost, try boost::array.
Author
Quote:Original post by rip-off
You cannot pass c-style arrays by value. You can either make them const to prevent accidental manipulation, or make them in a small structure and pass that by value.
Or just use std::vector for all you array-like needs.
If you have boost, try boost::array.
Thanks for the clarification.
You can pass them by reference to keep sizing information:
Although, you would be better off using std::vector, boost::array or boost::multi_array, as rip-off mentioned.
template< class T, size_t D1, size_t D2 >void Do( T ( &arr )[D1][D2] ) { arr[D1 / 2][D2 / 2] = 1;}int main() { int arr[4][6] = { 0 }; Do( arr );}Although, you would be better off using std::vector, boost::array or boost::multi_array, as rip-off mentioned.
Here is how I would approach passing by value:
#include <iostream>template <class T, int s>class Array{ T array;<br><br><span class="cpp-keyword">public</span>:<br><br> Array(<span class="cpp-keyword">const</span> T t)<br> {<br> <span class="cpp-keyword">for</span>(<span class="cpp-keyword">int</span> i=<span class="cpp-number">0</span>; i<s; i++)<br> array<span style="font-weight:bold;"> = t<span style="font-weight:bold;">;<br> }<br><br> Array(<span class="cpp-keyword">const</span> Array& t)<br> {<br> <span class="cpp-keyword">for</span>(<span class="cpp-keyword">int</span> i=<span class="cpp-number">0</span>; i<s; i++)<br> array<span style="font-weight:bold;"> = t<span style="font-weight:bold;">;<br> }<br><br> T& <span class="cpp-keyword">operator</span>[](<span class="cpp-keyword">int</span> i)<br> {<br> <span class="cpp-keyword">return</span> array<span style="font-weight:bold;">;<br> }<br><br> <span class="cpp-keyword">const</span> T& <span class="cpp-keyword">operator</span>[](<span class="cpp-keyword">int</span> i) <span class="cpp-keyword">const</span><br> {<br> <span class="cpp-keyword">return</span> array<span style="font-weight:bold;">;<br> }<br>};<br><br><span class="cpp-keyword">template</span> <<span class="cpp-keyword">class</span> T, <span class="cpp-keyword">int</span> s><br>std::ostream& <span class="cpp-keyword">operator</span><<(std::ostream& str, <span class="cpp-keyword">const</span> Array<T,s>& a)<br>{<br> <span class="cpp-keyword">for</span>(<span class="cpp-keyword">int</span> i=<span class="cpp-number">0</span>; i<<span class="cpp-number">4</span>; i++)<br> str << a<span style="font-weight:bold;"> << <span class="cpp-literal">" "</span>;<br><br> <span class="cpp-keyword">return</span> str;<br>}<br><br><span class="cpp-comment">//An equivalent of memset, passes arrays by value</span><br><span class="cpp-keyword">template</span> <<span class="cpp-keyword">class</span> T, <span class="cpp-keyword">int</span> s><br><span class="cpp-keyword">void</span> MemSetByVal(Array<T, s> a, T v)<br>{<br> <span class="cpp-keyword">for</span>(<span class="cpp-keyword">int</span> i=<span class="cpp-number">0</span>; i<s; i++)<br> a<span style="font-weight:bold;"> = v;<br> <span class="cpp-keyword">return</span>;<br>}<br><br><span class="cpp-comment">//An equivalent of memset, passes arrays by reference</span><br><span class="cpp-keyword">template</span> <<span class="cpp-keyword">class</span> T, <span class="cpp-keyword">int</span> s><br><span class="cpp-keyword">void</span> MemSetByRef(Array<T, s> &a, T v)<br>{<br> <span class="cpp-keyword">for</span>(<span class="cpp-keyword">int</span> i=<span class="cpp-number">0</span>; i<s; i++)<br> a<span style="font-weight:bold;"> = v;<br> <span class="cpp-keyword">return</span>;<br>}<br><br><span class="cpp-keyword">int</span> main()<br>{<br> <span class="cpp-keyword">int</span> arrayInit[] = {<span class="cpp-number">1</span>, <span class="cpp-number">2</span>, <span class="cpp-number">3</span>, <span class="cpp-number">4</span>};<br> Array<<span class="cpp-keyword">int</span>, <span class="cpp-number">4</span>> a = arrayInit;<br><br> MemSetByVal(a, <span class="cpp-number">0</span>);<br><br> std::cout << a << <span class="cpp-literal">"\n"</span>;<br><br> MemSetByRef(a, <span class="cpp-number">0</span>);<br><br> std::cout << a << <span class="cpp-literal">"\n"</span>;<br>}<br><br><br></pre></div><!–ENDSCRIPT–><br><br>It should print <br>1 2 3 4<br>0 0 0 0<br><br>because MemSetByVal doesn't do anything to the array, but MemSetByRef does. boost::array already does all of that, but if you don't want to use boost, there is nothing wrong with writing a custom array wrapper, at least you will get to practice writing templates.This topic is closed to new replies.
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