error: unused variable (struct)

Question

error: unused variable 'part2' [-Werror,-Wunused-variable]

The error occurs only for part2 even though it's been initialized as well, just in a different manner. Is it just a compiler issue ?

int main(void)
{
    struct complex
    {
        int a;
        int b;
    };

    struct complex part1;
    part1.a = 2;
    part1.b = 3;

    struct complex part2 = {4, 5};

    struct complex part3 = {part3.a = 7, part3.b = 8};
}   

Answer 1

As has been mentioned in the comments above, part1 is considered "used" because you assign values to its fields (a and b) explicitly in the following lines:

part1.a = 2;
part1.b = 3;

part2 is never used, only initialized in the line:

struct complex part2 = {4, 5};

part3 is more interesting, and I'm surprised your compiler is not throwing an error. By doing the following:

struct complex part3 = {part3.a = 7, part3.b = 8};

you are first assigning part3.a = 7 and part3.b = 8, then the results of those assignments are being used to initialize the structure (part3.a = 7 evaluates to 7, part3.b = 8 evaluates to 8). It essentially becomes the following set of statements:

part3.a = 7;
part3.b = 8;
struct complex part3 = {7, 8};

I would expect your compiler to throw an error because you are attempting to assign values to the fields of part3 before you have instantiated it. This is undefined behavior.