why calling a function without parameter isn't an error

Question

I mistakenly called a function which expects an integer and returns an integer without any parameter. I do not know whether it was even a function call or not. Here is the code

int foo(int a);
int main(){
    cout<<foo<<endl;
    return 0;
}
int foo(int a){
    return 100;
}

output is 1
What happened exactly?