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Re: conversion problem with template, const with gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5)


On Mon, Jun 14, 2004 at 04:04:18PM -0400, Roy Yves wrote:
> Hi:

Hello Roy,

> 
> I am really puzzled with errors like the following:
> 
> g++  -c pymat.cpp -o pymat.o -I/usr/local/matlab6p5/extern/include -I/usr/include/python2.2/numarray -I/usr/include/python2.2 
> pymat.cpp: In function `mxArray* makeMxFromNumeric(const PyArrayObject*)':
> pymat.cpp:216: invalid conversion from `const maybelong*' to `int*'
> pymat.cpp:216:   initializing argument 5 of `void copyNumeric2Mx(T*, int, int, 
>    double*, int*) [with T = char]'
> 
> where copyNumeric2Mx is a template function:
> 
> template <class T>
> void copyNumeric2Mx(T *pSrc, int pRows, int pCols, double *pDst, int *pStrides)
> { // ... }
> 
> for each instantiated template function like in the following:
>   case PyArray_CHAR:
>         copyNumeric2Mx((char *)(pSrc->data), lRows, lCols, lR, pSrc->strides);

It seems pSrc->strides is a "pointer to const maybelong". Whatever kind of type `maybelong' may be, 
C++ forbids to assign a constant object (via a pointer) to a variable as this would allow to change
the constant's value.

Either declare copyNumeric2Mx like that:

template <class T>
void copyNumeric2Mx(T *, int, int, double*, const int*);
                                            ^^^^^

(in case you're allowed to change the function's prototype)


OR you may "cast away" the object's constness:

  copyNumeric2Mx((char *)(pSrc->data), lRows, lCols, lR, const_cast<maybelong*>(pSrc->strides));

-- 
Claudio


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