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In C++ (and C++11), classes defining a + and += operators often define a - and -= operators that do nearly the same thing (except + is replaced with - in the function).

What is the best way to avoid duplicated code here (and still achieve good performance)? I am sure there is a good way to do it with functors:

Here's what I've tried:

#include <iostream>
#include <functional>

struct num {
  int val;

  const num & operator +=(const num & rhs) {
    return op_equals<std::plus<int> >(rhs);
  const num & operator -=(const num & rhs) {
    return op_equals<std::minus<int> >(rhs);

  template <typename op>
  const num & op_equals(const num & rhs) {
    op operation;
    val = operation(val, rhs.val);
    return *this;

int main() {
  num x{1};
  num y{2};
  x += y;

  std::cout << x.val << std::endl;
  return 0;

Note that this is a simple example; in reality the code in the += and -= operators is more complex.

Also, I am not married to functors in the solution-- any suggestions or advice?

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migrated from Jun 28 '12 at 2:12

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Have you looked at Boost.Operators? – ildjarn Jun 27 '12 at 21:52
Is replacing duplicated code with more code in total a win? – Jon Jun 27 '12 at 21:52
A common pattern is to implement operator+ in terms of operator+=, which minimises repeated code. – Oli Charlesworth Jun 27 '12 at 21:53
@Oliver: So using Boost is bad practice? – Niklas B. Jun 27 '12 at 21:53
I think in this particular case, KISS is the best practice. – Stephen Chu Jun 27 '12 at 21:54

I actually quite like this pattern and use it relatively often. I'm not sure if there is something better than functors for this - if there is, I'm not really aware of it. That being said, with C++11, I'd write this in a slightly different way:

struct num {
  int val;

  const num& operator +=(const num & rhs) {
    return op_equals(rhs, std::plus<int>());

  const num& operator -=(const num & rhs) {
    return op_equals(rhs, std::minus<int>());

  template <typename op>
  const num & op_equals(const num & rhs, op&& o) {
    val = o(val, rhs.val);
    return *this;

Note the rvalue reference (which is actually a "universal reference", so can bind to either an lvalue or rvalue reference), and the movement of op to a parameter - I don't really like having to specify it as a template parameter which then gets instantiated and called in the function. I think this way is slightly cleaner.

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+1 Nice suggestions. I know to some people this feels like overkill, but to me it feels elegant (no duplicated code = no finding errors twice...) – user Feb 21 '13 at 16:03

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