constructor with/without semicolon

for the following code, line L can also be written as
A(int i):data(i){};             //   L1
A(int i):data(i){}              //   L2

As I know, L2 works because it's a function;
but how can L1 work? It seems to be a declaration.

---code---
#include<iostream>
using namespace std;

class A{
	int data;
public:
	A(int i):data(i){}                   //  L
	void B(){
		cout<<data<<endl;
	}
};

int main(){
	A(3).B();
}

---code--