Add element to function in place C++

Question

Add element to function in place C++ Python code:

l1 = [1, 2, 3]
l2 = l1 + [4]
print(l2) #[1, 2, 3, 4]

l3 = l1+l2
print(l3) #[1, 2, 3, 1, 2, 3, 4]