Write a function that accepts a lambda expression

问题:

I have a method like this

template<typename T, typename U>
map<T,U> mapMapValues(map<T,U> old, T (f)(T,U))
{
    map<T,U> new;
    for(auto it = old.begin(); it != old.end(); ++it)
    {
        new[it->first] = f(it->first,it->second);
    }
    return new; 
}

and the idea is that you'd call it like this

BOOST_AUTO_TEST_CASE(MapMapValues_basic)
{
    map<int,int> test;
    test[1] = 1;
    map<int,int> transformedMap = VlcFunctional::mapMapValues(test, 
        [&](int key, int value) -> int
        {
            return key + 1; 
        }
    );
}

However I get the error: no instance of function template "VlcFunctional::mapMapValues" matches the argument list argument types are: (std::map, std::allocator>>, __lambda1)

Any idea what I'm doing wrong? Visual Studio 2008 and Intel C++ compiler 11.1

回答1:

Your function is expecting a function pointer, not a lambda.

In C++, there are, in general, 3 types of "callable objects".

1. Function pointers.
2. Function objects.
3. Lambda functions.

If you want to be able to use all of these in your function interface, then you could use std::function:

template<typename T, typename U> 
map<T,U> mapMapValues(map<T,U> old, std::function<T(T, U)> f)
{
    ...
}

This will allow the function to be called using any of the three types of callable objects above. However, the price for this convenience is a small amount of overhead on invokations on the function (usually a null pointer check, then a call through a function pointer). This means that the function is almost certainly not inlined (except maybe with advanced WPO/LTO).

Alternatively, you could add an additional template parameter to take an arbitrary type for the second parameter. This will be more efficient, but you lose type-safety on the function used, and could lead to more code bloat.

template<typename T, typename U, typename F> 
map<T,U> mapMapValues(map<T,U> old, F f) 

回答2:

Your parameter type declaration T (f)(T,U) is of type 'free function taking a T and a U and returning a T'. You can't pass it a lambda, a function object, or anything except an actual function with that signature.

You could solve this by changing the type of the parameter to std::function<T(T,U)> like this:

template<typename T, typename U> 
map<T,U> mapMapValues(map<T,U> old, std::function<T(T,U)>)
{
}

Alternately, you could declare the function type as a template argument like this:

template<typename T, typename U, typename Fn> 
map<T,U> mapMapValues(map<T,U> old, Fn fn)
{
  fn(...);
}

回答3:

I would like to contribute this simple but self-explanatory example. It shows how to pass "callable things" (functions, function objects, and lambdas) to a function or to an object.

// g++ -std=c++11 thisFile.cpp

#include <iostream>
#include <thread>

using namespace std;

// -----------------------------------------------------------------
class Box {
public:
  function<void(string)> theFunction; 
  bool funValid;

  Box () : funValid (false) { }

  void setFun (function<void(string)> f) {
    theFunction = f;
    funValid = true;
  }

  void callIt () {
    if ( ! funValid ) return;
    theFunction (" hello from Box ");
  }
}; // class

// -----------------------------------------------------------------
class FunClass {
public:
  string msg;
  FunClass (string m) :  msg (m) { }
  void operator() (string s) {
    cout << msg <<  s << endl; 
  }
};

// -----------------------------------------------------------------
void f (string s) {
  cout << s << endl;
} // ()

// -----------------------------------------------------------------
void call_it ( void (*pf) (string) ) {
  pf( "call_it: hello");
} // ()

// -----------------------------------------------------------------
void call_it1 ( function<void(string)> pf ) {
  pf( "call_it1: hello");
} // ()

// -----------------------------------------------------------------
int main() {

  int a = 1234;

  FunClass fc ( " christmas ");

  f("hello");

  call_it ( f );

  call_it1 ( f );

  // conversion ERROR: call_it ( [&] (string s) -> void { cout << s << a << endl; } );

  call_it1 ( [&] (string s) -> void { cout << s << a << endl; } );

  Box ca;

  ca.callIt ();

  ca.setFun (f);

  ca.callIt ();

  ca.setFun ( [&] (string s) -> void { cout << s << a << endl; } );

  ca.callIt ();

  ca.setFun (fc);

  ca.callIt ();

} // ()

回答4:

Lambda expressions with empty capture list should decay to function pointers, according to n3052. However it seems that this feature is not implemented in VC++ and only partially in g++, see my SO question.

回答5:

Here is some example of how to pass a function as parameter

class YourClass
{
void YourClass::callback(void(*fptr)(int p1, int p2))
{
    if(fptr != NULL)
      fptr(p1, p2);
}
};

void dummyfunction(int p1, int p2)
{
   cout << "inside dummyfunction " << endl;
}

YourClass yc;

// using a dummyfunction as callback
yc.callback(&dummyfunction);

// using a lambda as callback
yc.callback( [&](int p1, int p2) { cout << "inside lambda callback function" << endl; } );

// using a static member function 
yc.callback( &aClass::memberfunction );