Why must I put a semicolon at the end of class declaration in C++?
because you can optionally declare objects
class Thing
{
...
}instanceOfThing;
for historical reasons
This is why...
int a,b,c,d;
int main(void) {
struct y {
}; a, b, c, d;
struct x {
} a, b, c, d;
}
Two different statements, two completely different meanings, both legal C / C++, and the only difference is the ;
after the struct declaration.
The statement a, b, c, d;
by itself, in this context, just evaluates a
, b
, c
and d
. In this context, that does nothing.
However, if it's right after the struct/class definition (before the ;
) it creates 4 instances of the created struct/class, a
, b
, c
and d
A good rule to help you remember where to put semicolons:
- If it's a definition, it needs a semicolon at the end. Classes, structs and unions are all information for the compiler, so need a trailing ; to mark no declared instances.
- If it contains code, it doesn't need a semicolon at the end. If statements, for loops, while loops and functions contain code, so don't need a trailing ;.
Namespaces also don't require a trailing semicolon, because they can contain a mix of both the above (so can contain code, so don't need a semicolon).
The full syntax is, essentially,
class NAME { constituents } instances ;
where "constituents" is the sequence of class elements and methods, and "instances" is a comma-separated list of instances of the class (i.e., objects).
Example:
class FOO {
int bar;
int baz;
} waldo;
declares both the class FOO and an object waldo.
The instance sequence may be empty, in which case you would have just
class FOO {
int bar;
int baz;
};
You have to put the semicolon there so the compiler will know whether you declared any instances or not.
This is a C compatibility thing.