How I can pass callable object to function as parameter

Question

In c++ standard library almost all algo function takes a callable object as a argument. Now I want to try this thing with my program. I opened the the headers for function like find_if or search_n() but could not understand much about how these callable object parameters are handled and off course how argument is passed to them especially for lambda object ( can bind() be used for lambdas, I dont know)

Can any one explain me how this thing works. Thanks in advance

Depending on what you need, the easiest way is a template parameter. — chris, Mar 05 '13 at 23:47
say I want to write replica of find_if(). would you please explain the code? — deeiip, Mar 05 '13 at 23:48
You pass it to the function as an argument then the function calls it. That's all really. — Jonathan Wakely, Mar 05 '13 at 23:51

Joseph Mansfield · Accepted Answer · 2013-03-05T23:55:45.793

14

Just have a template parameter for the function-like type and take an argument of that type:

template <typename Function>
void foo(Function func) {
  // Use func like so
  func(10);
}

This is how the standard library algorithms do it. You could call it with a lambda like so:

foo([](int x) { std::cout << (x * 2) << std::endl; });

Of course, this requires that you specify in your documentation what kind of function you expect as the Function type (until we get (until we get Concepts). Should it be unary? Binary? What type arguments should it take? What should it return?

Alternatively, you can take a function of a specific type by using std::function:

int foo(std::function<int(char)> func) {
  return func('a');
}

This time, foo will only take function-like objects that take a char argument and return an int. There is one downside to this method, however, which is that the compiler is unlikely to inline any lambdas you pass as func.

The most basic way to take a function as an argument is with a function pointer:

int foo(int (*func)(char)) {
  return func('a');
}

However, this will only take function pointers (some lambdas can be converted to function pointers). It won't take functors or anything else of the sort.

edited Mar 05 '13 at 23:55

answered Mar 05 '13 at 23:49

Joseph Mansfield

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but how lambda is called inside those function. I cant call them by name – deeiip Mar 05 '13 at 23:51
@me.deeiip You call the argument of the function by its name `func`. – Joseph Mansfield Mar 05 '13 at 23:52
1

@me.deeiip, They're given a name when you name the parameter. – chris Mar 05 '13 at 23:52
2

@sftrabbit, Actually, as I learned not too long ago, function pointer parameters will take lambdas. Here's for your viewing pleasure: http://liveworkspace.org/code/4kafLJ%240 – chris Mar 05 '13 at 23:56
@chris you mean compiler gives a name to lambda. – deeiip Mar 05 '13 at 23:57
1

@sftrabbit, Ah, I just remembered the restriction: It can't capture anything. – chris Mar 05 '13 at 23:59
1

@me.deeiip Look at the first function I gave. When you pass a function as an argument, it is now named `func`. That is the argument name that is of the function type. You can call it like a function - because it *is* a function. – Joseph Mansfield Mar 05 '13 at 23:59
@chris Ah, I just found it in the standard. Very neat. – Joseph Mansfield Mar 06 '13 at 00:01

score 0 · Answer 2 · answered Mar 06 '13 at 00:02

All algorithm function must get some functions to test the algo like find_if have to know which predicate you want to test on the loop. All std lib works with template and you can pass your function test as callable object. Callable object are object that have overload operator(), with it, std lib will call your sent object with some parameter and get up a return value if necessary. You can find which parameters and return values that are needed on some site, specialy I always use http://www.cplusplus.com/ you can find many documentation on all std lib in reference menu.

Special exemple from you : http://www.cplusplus.com/reference/algorithm/find_if/ find_if will take 2 iterators, one as first and an other as last, and a callable object that will take a object pointed by your iterator and return a bool.

How I can pass callable object to function as parameter

2 Answers2