One of the issues we observed in "particletest.c" was that the declaration of a variable that references a structure must use the struct keyword as well. For instance, let us take a look at the declaration of particle:

···
struct Particle particle = { 0 };
···

To overcome this limitation, C allows us to declare new types by using the typedef keyword. This way, we have updated "particletest.c" to make the code more elegant and readable:

···
typedef struct
{
float x, y, z;
} Position;

typedef struct
{
Position position;
} Particle;

int main()
{
Particle particle = { 0 };
particle.position.x = 2;
particle.position.y = 1;
···
}

Given the fact that we are not specifying a tag anymore in the structure, is this code still correct?

Question

One of the issues we observed in "particletest.c" was that the declaration of a variable that references a structure must use the struct keyword as well. For instance, let us take a look at the declaration of particle:

···
struct Particle particle = { 0 };
···

To overcome this limitation, C allows us to declare new types by using the typedef keyword. This way, we have updated "particletest.c" to make the code more elegant and readable:

···
typedef struct
{
    float x, y, z;
} Position;

typedef struct
{
   Position position;
} Particle;

int main()
{
    Particle particle = { 0 };
    particle.position.x = 2;
    particle.position.y = 1;
    ···
}

Given the fact that we are not specifying a tag anymore in the structure, is this code still correct?

BlackTom AI · Accepted Answer

Let's walk through the code conceptually and check what the typedefs do for us, and then evaluate the initialization and member access.

- The code defines two typedefs:
  - typedef struct { float x, y, z; } Position;  // Position becomes a named type for an anonymous struct with three floats
  - typedef struct { Position position; } Particle;  // Particle becomes a named type for an anonymous struct containing a Position
  These typedefs allow us to use Position and Particle as if they were regular types, without ever naming the struct tags. This is valid C and is a common technique to avoid tag names.

- In main, the declaration Particle particle = { 0 }; initializes a Particle object. Since Particle has a single member named position of type Position, this initializer is providing an initializer for that first (and only) member.
  The key question is whether { 0 } is a valid initializer for a Particle when its first member is itself a struct (Position). In C, a scalar initializer (like 0) can initialize a scalar member, but a struct member requires a braced list to initialize its fields. However, when an outer aggregate has fewer initializer elements than its members, the remaining members are zero-initialized. In this case, the outer aggregate has one member (position); providing { 0 } is interpreted as initializing that first member with a zero value, and since position is a struct, the standard allows the inner struct to be zero-initialized as part of the outer initializer, effectively setting position.x, position.y, and position.z to 0.
  Therefore, Particle particle = { 0 }; is a valid, portable way to zero-initialize the entire Particle object, including the nested Position struct, in compliant C implementations.

- After initialization, the code sets particle.position.x and particle.position.y. Given that particle.position was zero-initialized, these assignments simply overwrite the relevant fields and demonstrate accessing a member of a nested struct via the typedef-named types. This usage is correct and consistent with the declared types.

- A potential pitfall people worry about is the absence of a tag name for the structs. That concern is unfounded here because the typedefs define named types (Position and Particle) that can be used without needing a struct tag. The code compiles and behaves as intended on standard-conforming C compilers that support standard C grammar for typedefs of anonymous structs.

- In summary, the absence of a struct tag does not invalidate the code; the typedefs provide named types for the anonymous structs, and the initialization pattern Particle particle = { 0 }; properly zero-initializes the entire Particle object, including its nested Position member. Therefore, the statement is consistent with C semantics in this usage.

DD2360/FDD3360 HT25 (appgpu25) Quiz 1: Basic knowledge 测验 1：基础知识

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