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diff --git a/content/projects/C_const.md b/content/projects/C_const.md new file mode 100644 index 0000000..9fe86ef --- /dev/null +++ b/content/projects/C_const.md @@ -0,0 +1,107 @@ ++++ +title = "C: the const modifier" +date = 2024-08-23 + +[extra] +latex = true ++++ + +--- + +### The `C` `const` modifier with nested pointers + +The `const` is a way to tell the `C` compiler to consider the target variable read-only. +On function declaration, the data arguments are _de facto_ `const`, but we could specify the pointers' behavior as seen below. + +_The key is to apply the `const` modifier from right to left_ + +```C +/* August 2024, C99. + * Const modifier to data pointers. + * Note: the patterns are just layers of Pascal's binomial triangle, + * easy to repete for a growing number of stars. + */ + +/* + * Zero star. + */ +/* Zero const modifier -> 1 choose 0. */ +static int data; +/* One const modifier -> 1 choose 1. */ +static int const constData; + +/* + * One star + */ +/* Zero const modifier -> 2 choose 0. */ +static int *ptr_data; +/* One const modifier -> 2 choose 1. */ +static int const *ptr_constData; +static int *const constPtr_data; +/* Two const modifiers -> 2 choose 2. */ +static int const *const constPtr_constData; + +/* + * Two stars + */ +/* Zero const modifier -> 3 choose 0. */ +static int **ptr_ptr_data; +/* One const modifier -> 3 choose 1. */ +static int const **ptr_ptr_constData; +static int *const *ptr_constPtr_Data; +static int **const constPtr_ptr_Data; +/* Two const modifiers -> 3 choose 2. */ +static int const *const *ptr_constPtr_constData; +static int const **const constPtr_ptr_constData; +static int *const *const constPtr_constPtr_data; +/* Three const modifiers -> 3 choose 3. */ +static int const *const *const constPtr_constPtr_constData; + +/* + * Three stars + */ +/* Zero const modifier -> 4 choose 0. */ +static int ***ptr_ptr_ptr_data; +/* One const modifier -> 4 choose 1. */ +static int const ***ptr_ptr_ptr_constData; +static int *const **ptr_ptr_constPtr_data; +static int **const *ptr_constPtr_ptr_data; +static int ***const constPtr_ptr_ptr_data; +/* Two const modifiers -> 4 choose 2. */ +static int const *const **ptr_ptr_constPtr_constData; +static int const **const *ptr_constPtr_ptr_constData; +static int const ***const constPtr_ptr_ptr_constData; +static int *const *const *ptr_constPtr_constPtr_data; +static int *const **const constPtr_ptr_constPtr_data; +static int **const *const constPtr_constPtr_ptr_data; +/* Three const modifiers -> 4 choose 3. */ +static int const *const *const *ptr_constPtr_constPtr_constData; +static int const *const **const constPtr_ptr_constPtr_constData; +static int const **const *const constPtr_constPtr_ptr_constData; +static int *const *const *const constPtr_constPtr_constPtr_data; +/* Four const modifiers -> 4 choose 4. */ +static int const *const *const *const constPtr_constPtr_constPtr_constData; +``` + +We can then do the same for following stars depth, but as we can see the number of possibilities is combinatorial: for \\(n\\) stars-pointers we will have \\(2^{n + 1}\\) `const` flavours possible. + +You can verify the above assumptions by: + +```C +#include <stddef.h> + +void foo(int *ptr_data, int const *ptr_constData, int *const constPtr_data, int const *const constPtr_constData) +{ + *ptr_data = 0; // OK: modifies the pointed data + ptr_data = NULL; // OK: modifies the pointer + + *ptr_constData = 0; // Error! + ptr_constData = NULL; // OK: modifies the pointer + + *constPtr_data = 0; // OK: modifies the pointed data + constPtr_data = NULL; // Error! + + *constPtr_constData = 0; // Error! + constPtr_constData = NULL; // Error! +} +``` |