How to give an argument a default value determined by a function of the other arguments

You could instead use function overloading.

int f2(int n1) { return f2(n1, f1(n1)); } int f2(int n1, int n2) { // do stuff } 

One solution is function overloading as other answers has already suggested.

Other solution is to use boost::optional type for the optional argument:

int f2( int n1, boost::optional<int> n2) { int n2value = n2 != boost::none? n2.get() : f1(n1); //use n1 and n2value in the rest of the function! } 

boost::optional usually helps when you have more than one optional arguments, e.g:

int f(int a, boost::optional<X> b, boost::optional<Y> c, boost::optional<Z> d) { //etc } 

In such situation function overloading explodes, as the number of functions increases linearly with each additional optional parameter. Thankfully, C++ doesn't have named-parameter, otherwise it would increase exponentially instead of linearly. :-)

This might not be the good approach but you also can utilize templates like following:

Similar to the default compare functions in Associate STL Containers (map,set, etc.)

struct f1{ int operator() (int m) const { //definition for f1 goes here }; }; struct f3{ int operator() (int m) const { //definition for any other f3 goes here }; }; template < class fun = f1> int f2( int n1, const fun& f=fun() ) { int x=f(n1); //std::cout<<x<<std::endl; } int main() { f2<f3>(11); //Call using f3 f2(12); // Call using default f1 }