c - Passing A Mutable Matrix as Constant Without Warnings -
my main function generates matrix array of values "m", , array of pointers start of rows "m". want pass matrix subroutine such no values can modified, , such no row pointers can modified. i.e., subroutine must not alter matrix. such, pass pointer constant pointer constant value. works fine. expected error messages generated example below.
#include<stdio.h> #include<stdlib.h> void fun(double const * v, double const * const * m) { v = v; // allowed pointless v[0] = v[0]; // not allowed m = m; // allowed pointless m[0] = m[0]; // not allowed m[0][0] = m[0][0]; // not allowed } int main() { double *v = (double *)malloc(2*sizeof(double)); double *m = (double *)malloc(4*sizeof(double)); double **m = (double **)malloc(2*sizeof(double *)); m[0] = &m[0]; m[1] = &m[2]; fun(v,m); return 0; } error messages:
test.c: in function ‘fun’: test.c:7:2: error: assignment of read-only location ‘*v’ test.c:9:2: error: assignment of read-only location ‘*m’ test.c:10:2: error: assignment of read-only location ‘**m’ these expected. far.
the problem passing non-constant matrix generates following warning. i'm using gcc v4.5 no options.
test.c: in function ‘main’: test.c:22:2: warning: passing argument 2 of ‘fun’ incompatible pointer type test.c:4:6: note: expected ‘const double * const*’ argument of type ‘double **’ note passing vector "v" generates no such warnings.
my question this: can pass mutable matrix subroutine in such way cannot modified, without casting, , without compiler warnings?
this help:
void fun(double const * const v, double const * const * const m) .... the problem facing double const * not const pointer double, pointer const double. double const * == const double *.
however there still 1 comment: ordinal types const specifiers not used.
void fun(double const * v, double const * const * m) .... // allows change v or m, relaxes caller side edit: pointers made totally const... data point can't modified.
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