It initialises all fields of the POD structure to 0. Rationale: const SomeStruct init = {Value}; Initialises the first field of SomeStruct to Value, the rest of the structure to zero (I forget the section in the standard, but it’s there somewhere) Thus: const SomeOtherStruct init = {{Value}}; Initialises the first field of the first … Read more
Evidently this feature has been added to C11, but alas I don’t have access to a C compiler of recent vintage (>= GCC 4.6.2). typedef struct foo { int a; } foo; typedef struct bar { struct foo; int b; } bar; int main() { bar b; b.a = 42; b.b = 99; return 0; … Read more
Use the form which is most functionally useful for your program. Basically, this means if it’s useful for the value to be nil, then use a pointer. From a performance perspective, primitive numeric types are always more efficient to copy than to dereference a pointer. Even more complex data structures are still usually faster to … Read more
memset is practically never the right way to do it. And yes, there is a practical difference (see below). In C++ not everything can be initialized with literal 0 (objects of enum types can’t be), which is why in C++ the common idiom is some_struct s = {}; while in C the idiom is some_struct … Read more
is memset(&mystruct, 0, sizeof mystruct) same as mystruct = { 0 }; ? No. memset(&mystruct, 0, sizeof mystruct) ; … will tell the compiler to call a function that we expect will set during execution the data in mystruct to zero. mystruct = { 0 }; … will set tell the compiler set by itself … Read more
Before we start: Let me point out that a lot of the confusion around this syntax comes because in C and C++ you can use the = {0} syntax to initialize all members of a C-style array to zero! See here: https://en.cppreference.com/w/c/language/array_initialization. This works: // z has type int[3] and holds all zeroes, as: `{0, … Read more
Here is my version: #include <stdio.h> struct matrix { int rows; int cols; int **val; } a = { .rows=3, .cols=1, .val = (int*[3]){ (int[1]){1}, (int[1]){2}, (int[1]){3} } }, b = { .rows=3, .cols=4, .val = (int*[3]){ (int[4]){1, 2, 3, 4}, (int[4]){5, 6, 7, 8}, (int[4]){9,10,11,12} } }; void print_matrix( char *name, struct matrix *m … Read more
It’s not legal to have a struct that contains itself as a member. This is because a struct has fixed size, and it must be at least as large as the sum of the sizes of each of its members. Your type would have to have 8 bytes for the two floats, at least one … Read more
Initialization of flexible array member in this way is not allowed as per C standard. C11: 6.7.2.1 Structure and union specifiers (p20-21): 21 EXAMPLE 2 After the declaration: struct s { int n; double d[]; }; the structure struct s has a flexible array member d. […] 22 Following the above declaration: struct s t1 … Read more
Should I declare a Mystruct variable, define the properties of Mystruct, assign a pointer to it, and return the pointer Definitely not, because the variable defined in the function (in “auto” storage class) will disappear as the function exits, and you’ll return a dangling pointer. You could accept a pointer to a Mystruct (caller’s responsibility … Read more