std::function: To wrap any element that is callable

Simple usage

#include <iostream>
#include <functional>
std::function<void(int , const std::string&)> myFuncObj;
void theFunc(int i, const std::string& s)
{
    std::cout << s << ": " << i << std::endl;
}
int main(int argc, char *argv[])
{
    myFuncObj = theFunc;
    myFuncObj(10, "hello world");
}

std::function used with std::bind

Think about a situation where we need to callback a function with arguments. std::function used with std::bind gives a very powerful design construct as shown below.

class A
{
public:
    std::function<void(int, const std::string&)> m_CbFunc = nullptr;
    void foo()
    {
        if (m_CbFunc)
        {
            m_CbFunc(100, "event fired");
        }
    }

};

class B
{
public:
    B()
    {
        auto aFunc = std::bind(&B::eventHandler, this, std::placeholders::_1, std::placeholders::_2);
        anObjA.m_CbFunc = aFunc;
    }
    void eventHandler(int i, const std::string& s)
    {
        std::cout << s << ": " << i << std::endl;
    }

    void DoSomethingOnA()
    {
        anObjA.foo();
    }

    A anObjA;
};

int main(int argc, char *argv[])
{
     B anObjB;
     anObjB.DoSomethingOnA();
}

std::function with lambda and std::bind

#include <iostream>
#include <functional>

using std::placeholders::_1; // to be used in std::bind example

int stdf_foobar (int x, std::function<int(int)> moo)
{
    return x + moo(x); // std::function moo called
}

int foo (int x) { return 2+x; }

int foo_2 (int x, int y) { return 9*x + y; }

int main()
{
    int a = 2;

    /* Function pointers */
    std::cout << stdf_foobar(a, &foo) << std::endl; // 6 ( 2 + (2+2) )
    // can also be: stdf_foobar(2, foo)

    /* Lambda expressions */
    /* An unnamed closure from a lambda expression can be
     * stored in a std::function object:
     */
    int capture_value = 3;
    std::cout << stdf_foobar(a,
                             [capture_value](int param) -> int { return 7 + capture_value * param; })
              << std::endl;
    // result: 15 ==  value + (7 * capture_value * value) == 2 + (7 + 3 * 2)

    /* std::bind expressions */
    /* The result of a std::bind expression can be passed.
     * For example by binding parameters to a function pointer call:
     */    
    int b = stdf_foobar(a, std::bind(foo_2, _1, 3));
    std::cout << b << std::endl;
    // b == 23 == 2 + ( 9*2 + 3 )
    int c = stdf_foobar(a, std::bind(foo_2, 5, _1));
    std::cout << c << std::endl;
    // c == 49 == 2 + ( 9*5 + 2 )

    return 0;
}

`function` overhead

std::function can cause significant overhead. Because std::function has [value semantics][1], it must copy or move the given callable into itself. But since it can take callables of an arbitrary type, it will frequently have to allocate memory dynamically to do this.

Some function implementations have so-called “small object optimization”, where small types (like function pointers, member pointers, or functors with very little state) will be stored directly in the function object. But even this only works if the type is noexcept move constructible. Furthermore, the C++ standard does not require that all implementations provide one.

Consider the following:

//Header file
using MyPredicate = std::function<bool(const MyValue &, const MyValue &)>;

void SortMyContainer(MyContainer &C, const MyPredicate &pred);

//Source file
void SortMyContainer(MyContainer &C, const MyPredicate &pred)
{
    std::sort(C.begin(), C.end(), pred);
}

A template parameter would be the preferred solution for SortMyContainer, but let us assume that this is not possible or desirable for whatever reason. SortMyContainer does not need to store pred beyond its own call. And yet, pred may well allocate memory if the functor given to it is of some non-trivial size.

function allocates memory because it needs something to copy/move into; function takes ownership of the callable it is given. But SortMyContainer does not need to own the callable; it’s just referencing it. So using function here is overkill; it may be efficient, but it may not.

There is no standard library function type that merely references a callable. So an alternate solution will have to be found, or you can choose to live with the overhead.

Also, function has no effective means to control where the memory allocations for the object come from. Yes, it has constructors that take an allocator, but [many implementations do not implement them correctly… or even at all][2].

The function constructors that take an allocator no longer are part of the type. Therefore, there is no way to manage the allocation.

Calling a function is also slower than calling the contents directly. Since any function instance could hold any callable, the call through a function must be indirect. The overhead of calling function is on the order of a virtual function call.

Binding std::function to a different callable types

/*
 * This example show some ways of using std::function to call
 *  a) C-like function
 *  b) class-member function
 *  c) operator()
 *  d) lambda function
 *
 * Function call can be made:
 *  a) with right arguments
 *  b) argumens with different order, types and count
 */
#include <iostream>
#include <functional>
#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using namespace std::placeholders;



// simple function to be called
double foo_fn(int x, float y, double z)
{
  double res = x + y + z;
  std::cout << "foo_fn called with arguments: " 
            << x << ", " << y << ", " << z 
            << " result is : " << res 
            << std::endl;
  return res;
}

// structure with member function to call
struct foo_struct
{
    // member function to call
    double foo_fn(int x, float y, double z)
    {
        double res = x + y + z;
        std::cout << "foo_struct::foo_fn called with arguments: " 
                << x << ", " << y << ", " << z 
                << " result is : " << res 
                << std::endl;
        return res;
    }
    // this member function has different signature - but it can be used too
    // please not that argument order is changed too
    double foo_fn_4(int x, double z, float y, long xx)
    {
        double res = x + y + z + xx;
        std::cout << "foo_struct::foo_fn_4 called with arguments: " 
                << x << ", " << z << ", " << y << ", " << xx
                << " result is : " << res 
                << std::endl;
        return res;
    }
    // overloaded operator() makes whole object to be callable
    double operator()(int x, float y, double z)
    {
        double res = x + y + z;
        std::cout << "foo_struct::operator() called with arguments: " 
                << x << ", " << y << ", " << z 
                << " result is : " << res 
                << std::endl;
        return res;
    }
};


int main(void)
{
  // typedefs
  using function_type = std::function<double(int, float, double)>;

  // foo_struct instance
  foo_struct fs;
  
  // here we will store all binded functions 
  std::vector<function_type> bindings;

  // var #1 - you can use simple function
  function_type var1 = foo_fn;
  bindings.push_back(var1);
  
  // var #2 - you can use member function 
  function_type var2 = std::bind(&foo_struct::foo_fn, fs, _1, _2, _3);
  bindings.push_back(var2);
  
  // var #3 - you can use member function with different signature
  // foo_fn_4 has different count of arguments and types
  function_type var3 = std::bind(&foo_struct::foo_fn_4, fs, _1, _3, _2, 0l);
  bindings.push_back(var3);

  // var #4 - you can use object with overloaded operator() 
  function_type var4 = fs;
  bindings.push_back(var4);

  // var #5 - you can use lambda function
  function_type var5 = [](int x, float y, double z)
    {
        double res = x + y + z;
        std::cout << "lambda  called with arguments: " 
                << x << ", " << y << ", " << z 
                << " result is : " << res 
                << std::endl;
        return res;
    };
  bindings.push_back(var5);
    
  std::cout << "Test stored functions with arguments: x = 1, y = 2, z = 3" 
            << std::endl;
  
  for (auto f : bindings)
      f(1, 2, 3);
      
}

Live

Output:

Test stored functions with arguments: x = 1, y = 2, z = 3
foo_fn called with arguments: 1, 2, 3 result is : 6
foo_struct::foo_fn called with arguments: 1, 2, 3 result is : 6
foo_struct::foo_fn_4 called with arguments: 1, 3, 2, 0 result is : 6
foo_struct::operator() called with arguments: 1, 2, 3 result is : 6
lambda  called with arguments: 1, 2, 3 result is : 6

Storing function arguments in std::tuple

Some programs need so store arguments for future calling of some function.

This example shows how to call any function with arguments stored in std::tuple

#include <iostream>
#include <functional>
#include <tuple>
#include <iostream>

// simple function to be called
double foo_fn(int x, float y, double z)
{
   double res =  x + y + z;
   std::cout << "foo_fn called. x = " << x << " y = " << y << " z = " << z
             << " res=" << res;
   return res;
}

// helpers for tuple unrolling
template<int ...> struct seq {};
template<int N, int ...S> struct gens : gens<N-1, N-1, S...> {};
template<int ...S> struct gens<0, S...>{ typedef seq<S...> type; };

// invocation helper 
template<typename FN, typename P, int ...S>
double call_fn_internal(const FN& fn, const P& params, const seq<S...>)
{
   return fn(std::get<S>(params) ...);
}
// call function with arguments stored in std::tuple
template<typename Ret, typename ...Args>
Ret call_fn(const std::function<Ret(Args...)>& fn, 
            const std::tuple<Args...>& params)
{
    return call_fn_internal(fn, params, typename gens<sizeof...(Args)>::type());
}


int main(void)
{
  // arguments
  std::tuple<int, float, double> t = std::make_tuple(1, 5, 10);
  // function to call
  std::function<double(int, float, double)> fn = foo_fn;
  
  // invoke a function with stored arguments
  call_fn(fn, t);
}