Struct and union name collision
I wrote a simple test program and tried to use it:
union FLT { ... };
struct FLT { ... };
But the compiler complained about a name clash.
Since in C you need to use struct
and union
for your tag name, why do they clash?
I mean when we want to declare a variable of a type stuct FLT
with a name num
, we will use
struct FLT num;
and when we want to use a variable with a diffnum
type name union FLT
, we will use
union FLT diffnum;
So why couldn't the compiler tell them apart?
BTW I've tested both MinGW and VC if you need to know.
UPDATE
union FLOAT {
float value;
unsigned int bits;
unsigned char bytes[4];
};
struct FLOAT {
unsigned int sign;
unsigned int exponent;
unsigned int significand;
};
union FLOAT num;
struct FLOAT num_parts;
I have used this code to do floating point arithmetic tests.
source to share
They clash because all tagged tags share the same namespace for tags.
C11 6.2.3 Identity Identifier Spaces (N1570 Project)
If more than one declaration of a specific identifier is visible at any point in translation units, the syntactic context is ambiguous which refer to different objects. Thus, there is a separate name for spaces for different categories of identifiers:
- label names (ambiguous by the syntax of label declaration and use);
- structure, union and enumeration tags (ambiguous, using any of the keywords struct, union, or enum);
- members of structures or unions; each structure or union has a separate namespace for its members (the type of expression used to access an element via. or → is ambiguous);
- all other identifiers called regular identifiers (declared in regular declarators or enumeration constants).
source to share