Question

I’ve the code:

#include <iostream>

using namespace std;

auto fn = ([](int x){
       return [x](int y) {
          return x * y;
       };
});

int main() {

    int i = fn(2)(4); // 8

    cout << i << endl;

    return 0;
}

This code works fine.
However, I want to call a second function later like:

auto i = fn(2);

i(4); //error: 'i' cannot be used as a function

Are there any way to call the last function later and then bind with the first call?

Answer 1

The following works as expected

#include <iostream>

using namespace std;

auto fn = [](int x){
       return [x](int y) {
          return x * y;
       };
  };

int main() {

    auto i = fn(2)(4); // 8
    cout << i << endl;
    auto j = fn(2);
    cout << j(4) << endl;

    return 0;
}

ADD

By the way gcc 4.5 with -std=c++0x gives the following error if you use int instead of auto:

currying.cpp:17:17: error: cannot convert ‘<lambda(int)>::<lambda(int)>’ to ‘int’ in initialization
currying.cpp:19:16: error: ‘j’ cannot be used as a function

which is an “obvious” and useful information to get what’s going wrong.