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POTwo dimensional arrays allocation problem
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  1. POTwo dimensional arrays allocation problem
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    copied!<p>this is an interview question my friend was asked yesterday. The question was something like: will this program crash with an "access violation" error or not? I looked at it for a while and thought no, it won't. But actually trying this out in visual studio proved me wrong. I cannot figure out what happens here... or to be more precise, i know what happens, but do not understand WHY. The problem seems to be that the matrix2 array does not get allocated at all.</p> <p>Code below:</p> <pre><code>#include &lt;iostream&gt; #include &lt;ctime&gt; using namespace std; int** matrixAlloc( const int rows, const int cols ); void matrixAlloc( int** matrix, const int rows, const int cols ); void matrixDealloc( int** m, const int rows); void matrixPrint( const int* const * const m, const int rows, const int cols ); int main( int argc, char** argv ) { srand( (unsigned int)time( NULL ) ); int** matrix1 = matrixAlloc( 4, 5 ); matrixPrint( matrix1, 4, 5 ); matrixDealloc( matrix1, 4 ); int ** matrix2 = NULL; matrixAlloc( matrix2, 4, 5 ); matrixDealloc( matrix2, 4 ); // &lt;--- crash occurs here } int** matrixAlloc( const int rows, const int cols ) { int **matrix = new int *[ rows ]; for ( int i = 0; i &lt; rows; i++ ) { matrix[ i ] = new int[ cols ]; for ( int j = 0; j &lt; cols; j++ ) { matrix[ i ][ j ] = (rand() * 12347) % 10; } } return matrix; } void matrixAlloc( int** matrix, const int rows, const int cols ) { matrix = new int *[ rows ]; for ( int i = 0; i &lt; rows; i++ ) { matrix[ i ] = new int[ cols ]; for ( int j = 0; j &lt; cols; j++ ) { matrix[ i ][ j ] = (rand() * 12347) % 10; } } } void matrixDealloc( int** matrix, const int rows ) { for ( int i = 0; i &lt; rows; i++ ) { delete [] matrix[ i ]; } delete [] matrix; } void matrixPrint( const int* const * const matrix, const int rows, const int cols ) { for ( int i = 0; i &lt; rows; i++ ) { for ( int j = 0; j &lt; cols; j++ ) { cout &lt;&lt; matrix[ i ][ j ] &lt;&lt; " "; } cout &lt;&lt; endl; } cout &lt;&lt; endl; } </code></pre>
 

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