# mem_fun_t<Result, X>

Categories: functors, adaptors | Component type: type

### Description

`Mem_fun_t` is an adaptor for member functions. If `X` is some class with a member function `Result X::f()` (that is, a member function that takes no arguments and that returns a value of type `Result` [1]), then a `mem_fun_t<Result, X>` is a function object adaptor that makes it possible to call `f()` as if it were an ordinary function instead of a member function.

`Mem_fun_t<Result, X>`'s constructor takes a pointer to one of `X`'s member functions. Then, like all function objects, `mem_fun_t` has an `operator()` that allows the `mem_fun_t` to be invoked with ordinary function call syntax. In this case, `mem_fun_t`'s `operator()` takes an argument of type `X*`.

If `F` is a `mem_fun_t` that was constructed to use the member function `X::f`, and if `x` is a pointer of type `X*`, then the expression `F(x)` is equivalent to the expression `x->f()`. The difference is simply that `F` can be passed to STL algorithms whose arguments must be function objects.

`Mem_fun_t` is one of a family of member function adaptors. These adaptors are useful if you want to combine generic programming with inheritance and polymorphism, since, in C++, polymorphism involves calling member functions through pointers or references.

As with many other adaptors, it is usually inconvenient to use `mem_fun_t`'s constructor directly. It is usually better to use the helper function `mem_fun` instead.

### Example

    struct B {
      virtual void print() = 0;
    };

    struct D1 : public B {
      void print() { cout << "I'm a D1" << endl; }
    };

    struct D2 : public B {
      void print() { cout << "I'm a D2" << endl; }
    };

    int main()
    {
      vector<B*> V;

      V.push_back(new D1);
      V.push_back(new D2);
      V.push_back(new D2);
      V.push_back(new D1);

      for_each(V.begin(), V.end(), mem_fun(&B::print));
    }

### Definition

Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h.

### Template parameters

Parameter | Description | Default
--- | --- | ---
`Result` | The member function's return type. |
`X` | The class whose member function the `mem_fun_t` invokes. |

### Model of

Adaptable Unary Function

### Type requirements

- `X` has at least one member function that takes no arguments and that returns a value of type `Result`. [1]

### Public base classes

`unary_function<X*, Result>`

### Members

Member | Where defined | Description
--- | --- | ---
`argument_type` | Adaptable Unary Function | The type of the argument: `X*`
`result_type` | Adaptable Unary Function | The type of the result: `Result`
    Result operator()(X* x) const
| Unary Function | Function call operator. Invokes `x->f()`, where `f` is the member function that was passed to the constructor.
    explicit mem_fun_t(Result (X::*f)())
| `mem_fun_t` | See below.
    template <class Result, class X>
    mem_fun_t<Result, X>
    mem_fun(Result (X::*f)());
| `mem_fun_t` | See below.

### New members

These members are not defined in the Adaptable Unary Function requirements, but are specific to `mem_fun_t`.

Member | Description
--- | ---
    explicit mem_fun_t(Result (X::*f)())
| The constructor. Creates a `mem_fun_t` that calls the member function `f`.
    template <class Result, class X>
    mem_fun_t<Result, X>
    mem_fun(Result (X::*f)());
| If `f` if of type `Result (X::*)` then `mem_fun(f)` is the same as `mem_fun_t<Result, X>(f)`, but is more convenient. This is a global function, not a member function.

### Notes

[1] The type `Result` is permitted to be `void`. That is, this adaptor may be used for functions that return no value. However, this presents implementation difficulties. According to the draft C++ standard, it is possible to return from a `void` function by writing `return void` instead of just `return`. At present, however (early 1998), very few compilers support that feature. As a substitute, then, `mem_fun_t` uses partial specialization to support `void` member functions. If your compiler has not implemented partial specialization, then you will not be able to use `mem_fun_t` with member functions whose return type is `void`.

### See also

`mem_fun_ref_t`, `mem_fun1_t`, `mem_fun1_ref_t`

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