Boost.Function Tutorial

Boost.Function has two syntactical forms: the preferred form and the compatibility form. The tutorial is therefore split into two sections: the first section introduces Boost.Function using the preferred form and the second section introduces Boost.Function using a compatibility form that is available on all supported compilers. If you intend to write code to be compiled only on conforming compilers, use the preferred form; if compatibility with nonconforming compilers (e.g., Borland C++ 5.5.1 or Microsoft Visual C++ 6.0/7.0) is required, use the compatibility form. The compatibility form coincides with the older Boost.Function usage.

Preferred Syntactic Form: Basic Usage

A function wrapper is defined simply by instantiating the function class template with the desired return type and argument types, formulated as a C++ function type. Any number of arguments may be supplied, up to some implementation-defined limit (10 is the default maximum). The following declares a function object wrapper f that takes two int parameters and returns a float:

boost::function<float (int x, int y)> f;

By default, function object wrappers are empty, so we can create a function object to assign to f:

struct int_div { 
  float operator()(int x, int y) const { return ((float)x)/y; }; 
};

f = int_div();

Now we can use f to execute the underlying function object int_div:

std::cout << f(5, 3) >> std::endl;

We are free to assign any compatible function object to f. If int_div had been declared to take two long operands, the implicit conversions would have been applied to the arguments without any user interference. The only limit on the types of arguments is that they be CopyConstructible, so we can even use references and arrays:

boost::function<void (int values[], int n, int& sum, float& avg)> sum_avg;

void do_sum_avg(int values[], int n, int& sum, float& avg)
{
  sum = 0;
  for (int i = 0; i < n; i++)
    sum += values[i];
  avg = (float)sum / n;
}

sum_avg = &do_sum_avg;

Invoking a function object wrapper that does not actually contain a function object is a precondition violation, much like trying to call through a null function pointer. We can check for an empty function object wrapper by querying its empty() method or, more succinctly, by using it in a boolean context: if it evaluates true, it contains a function object target, i.e.,

if (f)
  std::cout << f(5, 3) << std::endl;
else
  std::cout << "f has no target, so it is unsafe to call" << std::endl;

We can clear out a function target using the clear() member function.

Free functions

Free function pointers can be considered singleton function objects with const function call operators, and can therefore be directly used with the function object wrappers:

  float mul_ints(int x, int y) { return ((float)x) * y; }
  f = &mul_ints;

Note that the & isn't really necessary unless you happen to be using Microsoft Visual C++ version 6.

Member functions

In many systems, callbacks often call to member functions of a particular object. This is often referred to as "argument binding", and is beyond the scope of Boost.Function. The use of member functions directly, however, is supported, so the following code is valid:

  struct X {
    int foo(int);
  };

  boost::function<int (X*, int)> f;
  f = &X::foo;
  
  X x;
  f(&x, 5);

Several libraries exist that support argument binding. Three such libraries are summarized below:

Boost.Bind. This library allows binding of arguments for any function object. It is lightweight and very portable.
The C++ Standard library. Using std::bind1st and std::mem_fun together one can bind the object of a pointer-to-member function for use with Boost.Function:
```
  struct X {
    int foo(int);
  };

  boost::function<int (int)> f;
  X x;
  f = std::bind1st(std::mem_fun(&X::foo), &x);

  f(5); // Call x.foo(5)
```
The Boost.Lambda library. This library provides a powerful composition mechanism to construct function objects that uses very natural C++ syntax. Lambda requires a compiler that is reasonably conformant to the C++ standard.

References to Functions

In some cases it is expensive (or semantically incorrect) to have Boost.Function clone a function object. In such cases, it is possible to request that Boost.Function keep only a reference to the actual function object. This is done using the ref and cref functions to wrap a reference to a function object:

  stateful_type a_function_object;
  boost::function<int (int)> f;
  f = ref(a_function_object);

  boost::function<int (int)> f2(f);

Here, f will not make a copy of a_function_object, nor will f2 when it is targeted to f's reference to a_function_object. Additionally, when using references to function objects, Boost.Function will not throw exceptions during assignment or construction.

Compatible Syntactic Form: Basic usage

A function wrapper is defined simply by instantiating a functionI class template with the desired return type and argument types, where I denotes the number of argument types. Any number of arguments may be supplied, up to some implementation-defined limit (10 is the default maximum). The following declares a function object wrapper f that takes two int parameters and returns a float:

boost::function2<float, int, int> f;