Faulty use of constructor of dynamic array

[ToniAz]

Hello everyone,

According to someone, one can initialize a dynamically created array in the following way:

unsigned int * vec;
// ... do something to vec
double * a = (double *) malloc(4*sizeof(double));
a = (double[3]){	(double[3]){0.0,10.0,20.0}[vec[0]],
					(double[3]){1.0,2.0,3.0}[vec[1]],
					(double[6]){-2.0,-1.0,0.0,1.0,2.0,3.0}[vec[2]]
					}; // NO C COMPILER ERROR
					
double 	b[3];
b = (double[3]){	(double[3]){0.0,10.0,20.0}[vec[0]],
					(double[3]){1.0,2.0,3.0}[vec[1]],
					(double[6]){-2.0,-1.0,0.0,1.0,2.0,3.0}[vec[2]]
					}; // C COMPILER GIVES ERROR	

While there is no compilation error for the first assignment, the memory a is pointing to seems to change, surprisingly to me. This seems to solve the problem though:

memcpy(a, (double[3]){	(double[3]){0.0,10.0,20.0}[vec[0]],
					(double[3]){1.0,2.0,3.0}[vec[1]],
					(double[6]){-2.0,-1.0,0.0,1.0,2.0,3.0}[vec[2]]
					},
                                        3*sizeof(double)); // NO C COMPILER ERROR 

What does the first assignment do and why does it cause memory to change later in the program?

[Disch]

What does the first assignment do and why does it cause memory to change later in the program?

double * a = (double *) malloc(4*sizeof(double));

'a' is a pointer. 'malloc' creates a new block of uninitialized memory, and returns a pointer to it. Let's call this block of memory 'foo'.

when you say a = malloc, you are saying "I want 'a' to point to 'foo'"

(double[3]){ ... }

Here, you are creating another block of (initialized) memory. Let's call this memory 'bar'

a = (double[3]){ ... }

Here, you are reassigning a. Therefore you are making a point to something else. IE, you are making it point to bar.


So you effectively allocate memory for foo... tell a to point to foo... then immediately have a stop pointing to foo and have it point to bar instead.

Foo is lost, and will be a memory leak, since nothing points to it anymore -- it will be impossible to free().

[ToniAz]

Thanks for the explanation!