A problem with higher order functions and lambdas in C++0x

Question

I have a program where I must print many STL vectors on the screen after doing some calculation on each component. So I tried to create a function like this:

template <typename a> 
void printWith(vector<a> foo, a func(a)){
  for_each(foo.begin(), foo.end(), [func](a x){cout << func(x) << " "; });
}

And then use it like this:

int main(){
  vector<int> foo(4,0);
  printWith(foo, [](int x) {return x + 1;});
  return 0;
}

Unfortunately, I'm having a compiling error about the type of the lambda expression I've put inside the printWith call:

g++ -std=gnu++0x -Wall -c vectest.cpp -o vectest.o
vectest.cpp: In function ‘int main()’:
vectest.cpp:16:41: error: no matching function for call to ‘printWith(std::vector<int>&, main()::<lambda(int)>)’
vectest.cpp:10:6: note: candidate is: void printWith()
make: *** [vectest.o] Error 1

Of course, if I do:

int sumOne(int x) {return x+1;}

then printWith(foo, sumOne); works as intended. I thought the type of a lambda expression would be the type of a function with the inferred return type. I also though that I could fit a lambda anywhere I could fit a normal function. How do I make this work?

Answer 1

The following works for me:

#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

template <typename a, typename F>
void printWith(vector<a> foo, F f){
  for_each(foo.begin(), foo.end(), [&](a x){cout << f(x) << " "; });
}

int main(){
  vector<int> foo = {1,2,3,4,5};
  printWith(foo, [](int x) {return x + 1;});
  std::cout << '\n';
  return 0;
}

Testing:

$ g++-4.5 -std=gnu++0x -Wall test.cpp
$ ./a.out                            
2 3 4 5 6 

---

Alternatively, you can exploit the fact that closure types with no lambda-capture can be implicitly converted to function pointers. This is closer to your original code and also cuts down on the number of instantiations of the function template (in the original solution you get a new instantiation every time you use the function template with a different function object type; note though that it doesn't matter much in this specific case since the printWith function is very short and most probably will be always inlined):

#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

template <typename a, typename b>
void printWith(const vector<a>& foo, b f(a)){
  for_each(foo.begin(), foo.end(), [=](a x){cout << f(x) << " "; });
}

int main(){
  vector<int> foo = {1,2,3,4,5};
  printWith<int, int>(foo, [](int x) {return x + 1;});
  std::cout << '\n';
  return 0;
}

Unfortunately, implicit conversion doesn't play very well with template argument deduction: as you can see, I had to specify template arguments in the call to printWith.

Another alternative is to use std::function. This also helps to minimize the number of template instantiations and works even for lambda expressions with lambda-capture, but has the same problems with template argument deduction:

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>

using namespace std;

template <typename a, typename b>
void printWith(const vector<a>& foo, std::function<b(a)> f){
  for_each(foo.begin(), foo.end(), [&](a x){cout << f(x) << " "; });
}

int main(){
  vector<int> foo = {1,2,3,4,5};
  int y = 1;
  printWith<int, int>(foo, [&](int x) { return x + y; });
  std::cout << '\n';
  return 0;
}