Unions
suggest changeUnions are very useful tools, but come with a few important caveats:
- It is undefined behavior, per the C++ standard, to access an element of a union that was not the most recently modified member. Although a lot of C++ compilers permit this access in well defined ways, these are extensions and cannot be guaranteed across compilers.
A std::variant
(since C++17) is like a union, only it tells you what it currently contains (part of its visible state is the type of the value it holds at a given moment: it enforces value access happening only to that type).
- Implementations do not necessarily align members of different sizes to the same address.
Basic Union Features
Unions are a specialized struct within which all members occupy overlapping memory.
union U {
int a;
short b;
float c;
};
U u;
//Address of a and b will be equal
(void*)&u.a == (void*)&u.b;
(void*)&u.a == (void*)&u.c;
//Assigning to any union member changes the shared memory of all members
u.c = 4.f;
u.a = 5;
u.c != 4.f;
Typical Use
Unions are useful for minimizing memory usage for exclusive data, such as when implementing mixed data types.
struct AnyType {
enum {
IS_INT,
IS_FLOAT
} type;
union Data {
int as_int;
float as_float;
} value;
AnyType(int i) : type(IS_INT) { value.as_int = i; }
AnyType(float f) : type(IS_FLOAT) { value.as_float = f; }
int get_int() const {
if(type == IS_INT)
return value.as_int;
else
return (int)value.as_float;
}
float get_float() const {
if(type == IS_FLOAT)
return value.as_float;
else
return (float)value.as_int;
}
};
Undefined Behavior
union U {
int a;
short b;
float c;
};
U u;
u.a = 10;
if (u.b == 10) {
// this is undefined behavior since 'a' was the last member to be
// written to. A lot of compilers will allow this and might issue a
// warning, but the result will be "as expected"; this is a compiler
// extension and cannot be guaranteed across compilers (i.e. this is
// not compliant/portable code).
}
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