# unary_compose

 Categories: functors, adaptors Component type: type

## Description

`Unary_compose` is a functors adaptor. If `f` and `g` are both AdaptableUnaryFunction, and if `g`'s return type is convertible to `f`'s argument type, then `unary_compose` can be used to create a function object `h` such that `h(x)` is the same as `f(g(x))`. [1] As with other function object adaptors, the easiest way to create a `unary_compose` is to use the helper function `compose1`. It is possible to call `unary_compose`'s constructor directly, but there is usually no reason to do so.

## Example

Calculates the negative of the sines of the elements in a vector, where the elements are angles measured in degrees. Since the C library function `sin` takes its arguments in radians, this operation is the composition of three operations: negation, `sin`, and the conversion of degrees to radians.

```vector<double> angles;
vector<double> sines;
const double pi = 3.14159265358979323846;
...
assert(sines.size() >= angles.size());
transform(angles.begin(), angles.end(), sines.begin(),
compose1(negate<double>(),
compose1(ptr_fun(sin),
bind2nd(times<double>(), pi / 180.))));
```

## Definition

Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h. The `unary_compose` class is an SGI extension; it is not part of the C++ standard.

## Template parameters

Parameter Description Default
`AdaptableUnaryFunction1` The type of the first operand in the function composition operation. That is, if the composition is written `f o g` [1], then `AdaptableUnaryFunction1` is the type of the function object `f`.
`AdaptableUnaryFunction2` The type of the second operand in the function composition operation. That is, if the composition is written `f o g` [1], then `AdaptableUnaryFunction1` is the type of the function object `g`.

## Type requirements

`AdaptableUnaryFunction1` and `AdaptableUnaryFunction2` must both be models of AdaptableUnaryFunction. `AdaptableUnaryFunction2::result_type` must be convertible to `AdaptableUnaryFunction1::argument_type`.

## Public base classes

```unary_function<AdaptableUnaryFunction2::argument_type,
```

## Members

Member Where defined Description
`argument_type` AdaptableUnaryFunction The type of the function object's argument: `AdaptableUnaryFunction2::argument_type`.
`result_type` AdaptableUnaryFunction The type of the result: `AdaptableUnaryFunction1::result_type`
```unary_compose(const AdaptableUnaryFunction1& f,
```
`unary_compose` See below.
```template <class AdaptableUnaryFunction1, class AdaptableUnaryFunction2>
```
`unary_compose` See below.

## New members

These members are not defined in the AdaptableUnaryFunction requirements, but are specific to `unary_compose`.

Member Description
```unary_compose(const AdaptableUnaryFunction1& f,
```
The constructor. Constructs a `unary_compose` object that represents the function object `f o g`. [1]
```template <class AdaptableUnaryFunction1, class AdaptableUnaryFunction2>
```
Creates a `unary_compose` object. If `f` and `g` are, respectively, of classes `AdaptableUnaryFunction1` and `AdaptableUnaryFunction2`, then `compose1(f, g)` is equivalent to `unary_compose<AdaptableUnaryFunction1, AdaptableUnaryFunction2>(f, g)`, but is more convenient. This is a global function, not a member function.

## Notes

[1] This operation is called function composition, hence the name `unary_compose`. It is often represented in mathematics as the operation `f o g`, where `f o g` is a function such that `(f o g)(x) == f(g(x))`. Function composition is a very important concept in algebra. It is also extremely important as a method of building software components out of other components, because it makes it possible to construct arbitrarily complicated function objects out of simple ones.

The functors, `binary_compose`, `binder1st`, `binder2nd`.