C++ 使用嵌套联合定义结构时需要一个表达式

Question

I have a struct with a nested union in c++, as follows:

typedef enum {            
  VAL_BOOL,               
  VAL_NIL, 
  VAL_NUMBER,             
} ValueType; 

typedef struct {  
  ValueType type; 
  union {         
    bool boolean; 
    double number;
  } as; 
} Value; 

I am trying to build a #define function to make initializing these values more readable, and this is what I have thus far, but it doesn't compile:

#define NUMBER_VAL(value) ((Value){ VAL_NUMBER, { .number = value } })

The above implementation generates the following error at compile-time when the NUMBER_VAL(val) is used: expected an expression

The goal here is to be able to define a Value by writing something like the following:

double dub = 1.23;
Value val = NUMBER_VAL(dub);

or pass it to a function, like so:

void process_value(Value value);
...
double dub = 45.6; 
process_value(NUMBER_VAL(dub));

Is there some way to convert this #define -ition into an expression that would let me use it this way? Or is my only option here to write a proper function to build these structs for me?

---

For the record, I know that the following would work, but I was hoping for something more succinct.

Value NUMBER_VAL(double value) {
    Value val;
    val.type = VAL_NUMBER;
    val.as.number = value;
    return val;
}

Answer 1

Live code .

In C++ unions can have constructors. In your struct you can define constructor overloads to initialize the union member along with the union tag ie type .