How do I set parameter default values that rely on other parameters?

Question

The following code compiles and works as expected.

#include 

void function(std::vector vec, int size=1);

int main(){
    std::vect         


        

Answer 1

Why do you need second parameter if it based on the first?

    void function(std::vector vec ){
        size_t size = vec.size();
        //code..
        return;
    }

Isn't it easier?