[ENH] Specify default values for C struct declarations

Is your feature request related to a problem? Please describe.

If there is a sturct with many fields, only some of its fields need to be initialized explicitly, and other fields can be implicitly empty-initialized, designators will be useful in c99:

struct MyStruct {
    int a;
    int b;
    int c;
    int d;
    float e;
    float f;
};

int main(void) {
    struct MyStruct s1 = {
        .a = 1,
        .b = 2,
    };
    struct MyStruct s2 = {
        .c = 42,
        .d = 233,
    };
    struct MyStruct s3 = {
        .e = 0.1,
        .f = 0.2,
    };
}

However, cython will leave other fields uninitialized and generate some warnings.

cdef struct MyStruct:
    int a
    int b
    int c
    int d
    float e
    float f

def main():
    cdef MyStruct s1 = MyStruct(
        a = 1,
        b = 2,
    )
    cdef MyStruct s2 = MyStruct(
        c = 42,
        d = 233,
    )
    cdef MyStruct s3 = MyStruct(
        e = 0.1,
        f = 0.2,
    )

warning: test.pyx:10:31: Not all members given for struct 'MyStruct'
warning: test.pyx:10:31: Not all members given for struct 'MyStruct'
warning: test.pyx:14:31: Not all members given for struct 'MyStruct'
warning: test.pyx:14:31: Not all members given for struct 'MyStruct'
warning: test.pyx:18:31: Not all members given for struct 'MyStruct'
warning: test.pyx:18:31: Not all members given for struct 'MyStruct'

The generated C code looks like this:

struct __pyx_t_4test_MyStruct __pyx_v_s2;
struct __pyx_t_4test_MyStruct __pyx_t_1;

/* "test.pyx":14
 *         b = 2,
 *     )
 *     cdef MyStruct s2 = MyStruct(             # <<<<<<<<<<<<<<
 *         c = 42,
 *         d = 233,
 */
__pyx_t_1.c = 42;
__pyx_t_1.d = 0xE9;
__pyx_v_s2 = __pyx_t_1;

Describe the solution you’d like.

It would be helpful if it’s possible to specify default values for the fields of C structures.

cdef struct MyStruct:
    int a = 0
    float b = 0
    void *c = NULL
    int d[4] = [0, 0, 0, 0]

# equivalent to above:

@cython.default_to_zero
cdef struct MyStruct:
    int a
    float b
    void *c
    int d[4]

def main():
    cdef int foo = 42
    cdef MyStruct s = MyStruct(
        b = 1.23,
        c = &foo,
    )

This code behaves the same as if specified a = 0 and d = [0, 0, 0, 0]:

def main():
    cdef int foo = 42
    cdef MyStruct s = MyStruct(
        a = 0,
        b = 1.23,
        c = &foo,
        d = [0, 0, 0, 0],
    )

The whole struct can also be empty-initialized:

cdef struct Inner:
    int a = 0
    int b = 0

cdef struct Outer:
    int a = 0
    int b = 0
    Inner c = Inner()

# equivalent to above:

@cython.default_to_zero
cdef struct Inner:
    int a
    int b

@cython.default_to_zero
cdef struct Outer:
    int a
    int b
    Inner c


def main():
    cdef Outer s = Outer()

Describe alternatives you’ve considered.

No response

Additional context

No response

[da-woods]

Definitely not opposed to this if someone does the work. Don’t think we can implement it in terms of designated initializers though.

[da-woods]

Due to C99 or due to other limitations?

Partly C++ too, which only as them as a c++20 feature.

But mostly that Cython’s code generation puts the declarations at the start of the function and the initialisation a little way in. That’s partly for C89 reasons, but mostly just so that variables are function scoped like Python