Is there a C++ equivalent to python's functools.partial

Question

I really like the functools.partial function in python and the idea of this functionallity in general. As example consider the following python script (I know that this case is not a very usefull example für using functools.partial, it should just be an easy example.)

import functools

def func(a, b, c):
    sum = a + b + c
    return sum


if __name__ == "__main__":
    func_p = functools.partial(func, a=1, c=1)
    sum = func_p(b=1)
    print(sum)

Is there something in C++ which offers a similar functionallity?

You're looking for lambdas or `std::bind`. (the latter is probably the most direct counterpart of `partial`) — Columbo, May 16 '15 at 10:12

Ami Tavory · Answer 1 · 2015-05-16T14:14:32.773

8

Yes, lambda functions:

auto func_p = [](int b){return func(1, b, 1);};

func_p(1);

Incidentally, I personally prefer lambdas in Python too. Consider the following:

lambda b: func(b**2, b, b - 3)

which can't be done with functools. Why have two different solutions (one applicable only in certain instances)?

There should be one-- and preferably only one --obvious way to do it.

edited May 16 '15 at 14:14

answered May 16 '15 at 10:02

Ami Tavory

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I gave your answer an upvote since it works; although have a look at this answer and you might rethink your general opinion on functools http://stackoverflow.com/a/3252425/4408275 – user69453 May 16 '15 at 14:18
@user69453 It **was** very interesting. However, on second thought, all his points seem relevant to Python, not C++. So if in Python it's a tradeoff, I still don't see much of a use for ``bind`` in C++. – Ami Tavory May 16 '15 at 14:25
yes the provided link was meant only for python. I just liked the syntax of `bind` more in c++. Thats all ;) – user69453 May 16 '15 at 14:28

score 4 · Accepted Answer · answered May 16 '15 at 10:41

A similar facility in C++ may be std::bind. See the following code for an illustration:

#include <functional>
#include <iostream>

int func(int a, int b, int c) {
  return a + b + c;
}

int main() {
  auto func_p = std::bind(func, 1, std::placeholders::_1, 1);
  std::cout << func_p(1) << std::endl;
}

Is there a C++ equivalent to python's functools.partial

2 Answers2