Avoiding if statement inside a for loop?

Question

I have a class called Writer that has a function writeVector like so:

void Drawer::writeVector(vector vec, bool index=true         


        

Answer 1

Pass in the body of the loop as a functor. It gets inlined at compile-time, no performance penalty.

The idea of passing in what varies is ubiquitous in the C++ Standard Library. It is called the strategy pattern.

If you are allowed to use C++11, you can do something like this:

#include 
#include 
#include 

template 
void for_each_indexed(const Container& c, Functor f, Index index = 0) {

    for (const auto& e : c)
        f(index++, e);
}

int main() {

    using namespace std;

    set s{'b', 'a', 'c'};

    // indices starting at 1 instead of 0
    for_each_indexed(s, [](size_t i, char e) { cout< v{77, 88, 99};

    // without index
    for_each_indexed(v, [](size_t , int e) { cout<

This code is not perfect but you get the idea.

In old C++98 it looks like this:

#include 
#include 
using namespace std;

struct with_index {
  void operator()(ostream& out, vector::size_type i, int e) {
    out << i << '\t' << e << '\n';
  }
};

struct without_index {
  void operator()(ostream& out, vector::size_type i, int e) {
    out << e << '\n';
  }
};


template 
void writeVector(const vector& v, Func f) {
  for (vector::size_type i=0; i v;
  v.push_back(77);
  v.push_back(88);
  v.push_back(99);

  writeVector(v, with_index());

  cout << "-----" << endl;

  writeVector(v, without_index());

  return 0;
}

Again, the code is far from perfect but it gives you the idea.