如何部分初始化结构的嵌套元素

Question

I'm having trouble initializing an element of a struct which is declared within another struct.

My structs look like this:

struct finger
{
  // other fields
  int pin;
};

struct Glove {
  struct finger index;
  struct finger middle;
  struct finger ring;
  struct finger pinkie;
  struct finger thumb;
};

//Initializing the array
struct Glove glove = { .index.pin = 20, .middle.pin = 22, .ring.pin = 24, .thumb.pin = 26};

What I'm trying to do is to only initialize the pin variable in the struct finger when declaring a glove struct.

However, I get an error message saying:

expected primary-expression before '.' token

My complete error messages:

Sensor_Glove:40:24: error: expected primary-expression before '.' token

 struct Glove glove = { .index.pin = 20, .middle.pin = 22, .ring.pin = 24, .thumb.pin = 26};

                        ^

Sensor_Glove:40:41: error: expected primary-expression before '.' token

 struct Glove glove = { .index.pin = 20, .middle.pin = 22, .ring.pin = 24, .thumb.pin = 26};

                                         ^

Sensor_Glove:40:59: error: expected primary-expression before '.' token

 struct Glove glove = { .index.pin = 20, .middle.pin = 22, .ring.pin = 24, .thumb.pin = 26};

                                                           ^

Sensor_Glove:40:75: error: expected primary-expression before '.' token

 struct Glove glove = { .index.pin = 20, .middle.pin = 22, .ring.pin = 24, .thumb.pin = 26};

                                                                           ^

exit status 1
expected primary-expression before '.' token

Answer 1

The syntax you have used is not valid for initialization of nested struct s. See here -> Nested initialization section - for valid syntax.

The following are valid alternatives for what you are trying to achieve.

Using designators ...

struct Glove glove = {                                                                                 
    .index = {
        .pin = 20
    },                                                                                                 
    .middle = {                                                                                        
        .pin = 22                                                                                      
    },
    .ring = {
        .pin = 24 
    },
    .thumb = {
        .pin = 26                                                                                      
    },                                                                                                 
};

Relying on the declaration order of the struct elements ...

struct Glove glove3 = {
    {20}, // index - pin 
    {22}, // middle - pin                                                                              
    {24}, // ring - pin                                                                                
    {0}, // pinkie - pin                                                                               
    {26}, // thumb - pin                                                                               
};

Using designators for outer elements, declaration order for inner ...

struct Glove glove2 = {                                                                                
    .index = {20}, // pin = 20                                                                         
    .middle = {22}, // pin = 22                                                                        
    .ring = {24}, // pin = 24                                                                          
    .thumb = {26}, // pin = 26                                                                         
    };

Being inconsistent ...

struct Glove glove4 = {
    .index = {20}, // index.pin = 20                                                                   
    {22}, // middle.pin = 22                                                                           
    // Since we are using designators we can change the order                                          
    .thumb = {26}, // thumb.pin = 26                                                                   
    .pinkie = {.pin = 24}, // pinkie.pin = 24                                                          
    };