Why structs cannot be assigned directly?

Question

Suppose I have a fully defined struct with tag MyStruct, and suppose that x, y, ..., z are allowed values for its fields. Why is

struct MyStruct q = {x,y,..,z}; 

allowed, but

struct MyStruct q; q = {x,y,...,z}; 

is not allowed? I find this very annoying. In the second case, where I have previously declared q, I need to assign a value to each field, one by one:

q.X = x; q.Y = y; ... q.Z = z; 

where X, Y, ..., Z are the fields of MyStruct. Is there a reason behind this?

Answer 1

What you are looking for is a compound literal. This was added to the language in C99.

Your first case:

struct MyStruct q = {x,y,..,z}; 

is a syntax specific to initialization. Your second case, in the pedantics of the language is not initialization, but assignment. The right hand side of the assignment has to be a struct of the correct type. Prior to C99 there was no syntax in the language to write a struct literal, which is what you are trying to do. {x,y,..,z} looked like a block with an expression inside. If one were inspired to try to think of it as a literal value, though the language didn't, one couldn't be sure of its type. (In your context, you could make a good guess.)

To allow this and resolve the type issue, C99 added syntax so you could write:

q = (struct MyStruct){x,y,...,z};