mag frontier -o cpp cpp In function int main cpp error too few argumen

[mag@frontier:~]% g++ -o a a.cpp
a.cpp: In function ‘int main()’:
a.cpp:1: error: too few arguments to function ‘int a(int)’
a.cpp:3: error: at this point in file
[mag@frontier:~]1% cat a.cpp 
int a(int b);

int main() { a(); }

int a(int b = 1) { }