error C2440

  • Thread starter Thread starter Allen Maki
  • Start date Start date
A

Allen Maki

Hi everybody,

Thanks for your suppoty.

//Why I got the following error message? for the 4th line:

// error C2440: '=' : cannot convert from

//'long (*)[3]' to 'long *' Types pointed

//to are unrelated; conversion requires

// reinterpret_cast, C-style cast or

//function-style cast





#include <iostream>

using namespace std;


int main()

{

const int max = 4; //(1)

const int max2 = 3; //(2)

long *pprime; //(3)

pprime = new long[max][max2]; //(4)


return 0;

}
 
Allen said:
// error C2440: '=' : cannot convert from

//'long (*)[3]' to 'long *' Types pointed
Click to expand...

That's because the left hand side doesn't match the type of the right
hand side.

The best way to use 2D arrays is to flatten them. Allocate a 1D array of
the size columns * rows:

long * pprime = new long[columns * rows];

and calculate the linearized index on your own, like this:

pprime[row * columns + column] = 0;

You need to take care of deleting your array when it's not used anymore:

delete [] pprime;

If you absolutely need the [row][column] syntax, or if the number of
columns is different in each row, then you have to allocate an array of
pointers, and then for each row allocate an array that holds the columns:

long** matrix = new long[rows];
for(int i = 0; i < rows; ++i)
matrix = new long[columns];

matrix[row][column] = whatever;

[...]

for(int i = 0; i < rows; ++i)
delete [] matrix;
delete [] matrix;

I strongly recommend that you use std::vector if you program in C++, and
use a 1D vector instead of a 2D one.

Tom
 
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