# More and More Utilities in C++20

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Today, I present a few utilities for calculating the midpoint of two values, check if a `std::string` starts or ends with a substring, and create callables with` std::bind_front`. These little utilities may not seem so little when you need them. Let's start arithmetical.

## Midpoint and Linear Interpolation

• `std::midpoint(a, b)` calculates the midpoint` (a + (b - a) / 2)` of the integers, floating-points, or pointers. If a and b are pointer, they have to point to the same array object.
• `std::lerp(a, b, t)` calculates the linear interpolation (a + t( b - a)). When t is outside the range [0, 1] it calculates the linear extrapolation.

The following program applies both functions.

```// midpointLerp.cpp

#include <cmath>     // std::lerp
#include <numeric>   // std::midpoint
#include <iostream>

int main() {

std::cout << std::endl;

std::cout << "std::midpoint(10, 20): " << std::midpoint(10, 20) << std::endl;

std::cout << std::endl;

for (auto v: {0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0}) {
std::cout << "std::lerp(10, 20, " << v << "): " << std::lerp(10, 20, v) << std::endl;
}

}
```

The output of the program should be self-explanatory. If not, try it out on the Compiler Explorer. C++20 has convenience functions for creating arrays.

## Creating Arrays and

With `std::to_array, `and `std::make_shared,` C++20 offers new  ways to create a `std::array` or `std::shared_ptr` from C-arrays.

### `std::to_array`

Thanks to` std::to_array`, creating a `std::array` from a C-array is a straightforward job.

```// toArray.cpp

#include <type_traits>
#include <utility>
#include <array>

int main(){

auto arr1 = std::to_array("C-String Literal");
static_assert(arr1.size() == 17);                  // (1)

auto arr2 = std::to_array({ 0, 2, 1, 3 });         // (2)
static_assert(std::is_same<decltype(arr2), std::array<int, 4>>::value);

auto arr3 = std::to_array<long>({ 0, 1, 3 });      // (3)
static_assert(std::is_same<decltype(arr3), std::array<long, 3>>::value);

auto arr4 = std::to_array<std::pair<int, float>>( { { 3, .0f }, { 4, .1f }, { 4, .1e23f } });
static_assert(arr4.size() == 3);                  // (4)
static_assert(std::is_same<decltype(arr4), std::array<std::pair<int, float>, 3>>::value);

}
```

The lines (1), (2), (3), and (3) assert that the created std::array has the expected type and size.

Per design, a `std::array` is as cheap and as fast as a C-array. If you want to know more about` std::array` and why you should not use a C-array, read my post "std::array - Dynamic Memory, no Thanks".

Additionally, a` std::array` knows its size and supports the typical interface of each container of the Standard Template Library, such as `std::vector`.

So far, all MSVC, Clang, GCC compiler support this convenient way to create a std::array. This observation does not hold for the next feature.

### `Create a std::shared_ptr of C-arrays`

Since C++11, C++ has the factory function `std::make_shared` to create a `std::shared_ptr`. Since C++20, `std::make_shared` also supports the creation of `std::shared_ptr` of C-arrays.

```auto s1 = std::make_shared<double[]>(1024);
auto s2 = std::make_shared<double[]>(1024, 1.0);
```

`s1` is a `std::shared_ptr `of a C-array. All members are default initialized. s2 is a `std::shared_ptr` of a C-array. Each element is initialized to `1.0.`

In contrast, the new two new member functions of `std::string` are already available with a brand-new MSVC, Clang, or GCC compiler.

## Check if a String starts with a Prefix or ends with a Suffix

`std::string` get a new member functions `starts_with` and` ends_with` which check if a `std::string` start or ends with a specified substring

```// stringStartsWithEndsWith.cpp

#include <iostream>
#include <string_view>
#include <string>

template <typename PrefixType>
void startsWith(const std::string& str, PrefixType prefix) {
std::cout << "            starts with " << prefix << ": "
<< str.starts_with(prefix) << '\n';    // (1)
}

template <typename SuffixType>
void endsWith(const std::string& str, SuffixType suffix) {
std::cout << "            ends with " << suffix << ": "
<< str.ends_with(suffix) << '\n';
}

int main() {

std::cout << std::endl;

std::cout << std::boolalpha;

std::string helloWorld("Hello World");

std::cout << helloWorld << std::endl;

startsWith(helloWorld, helloWorld);                 // (2)

startsWith(helloWorld, std::string_view("Hello"));  // (3)

startsWith(helloWorld, 'H');                        // (4)

std::cout << "\n\n";

std::cout << helloWorld << std::endl;

endsWith(helloWorld, helloWorld);

endsWith(helloWorld, std::string_view("World"));

endsWith(helloWorld, 'd');

}
```

Both member functions `starts_with` end `ends_with` are predicates. This means they return a boolean. You can invoke the member function` starts_with` (line 1) with a` std::string` (line 2), a `std::string_view` (line 3), and a `char` (line 4). The next utility function in C++20 my wonder you.

## std::bind_front

`std::bind_front (Func&& func, Args&& ... args`) creates a callable wrapper for a callable` func. std::bind_front` can have an arbitrary number of arguments and binds its arguments to the front.

Now, to the part which may wonder you. Since C++11, we have` std::bind` and lambda expression. To be pedantic` std::bind` is available since the Technical Report 1 (TR1). Both can be used as a replacement of` std::bind_front`. Furthermore, `std::bind_front` seems like the small sister of `std::bind,` because `std::bind` only supports the rearranging of arguments.Of course, there is a reason in the future to use `std::bind_front:` `std::bind_front` propagates exception specification of the underlying call operator.

The following program exemplifies, that you can replace `std::bind_front `with` ``std::bind,` or lambda expressions.

```// bindFront.cpp

#include <functional>
#include <iostream>

int plusFunction(int a, int b) {
return a + b;
}

auto plusLambda = [](int a, int b) {
return a + b;
};

int main() {

std::cout << std::endl;

auto twoThousandPlus1 = std::bind_front(plusFunction, 2000);         // (1)
std::cout << "twoThousandPlus1(20): " << twoThousandPlus1(20) << std::endl;

auto twoThousandPlus2 = std::bind_front(plusLambda, 2000);           // (2)
std::cout << "twoThousandPlus2(20): " << twoThousandPlus2(20) << std::endl;

auto twoThousandPlus3 = std::bind_front(std::plus<int>(), 2000);     // (3)
std::cout << "twoThousandPlus3(20): " << twoThousandPlus3(20) << std::endl;

std::cout << "\n\n";

using namespace std::placeholders;

auto twoThousandPlus4 = std::bind(plusFunction, 2000, _1);           // (4)
std::cout << "twoThousandPlus4(20): " << twoThousandPlus4(20) << std::endl;

auto twoThousandPlus5 =  [](int b) { return plusLambda(2000, b); };  // (5)
std::cout << "twoThousandPlus5(20): " << twoThousandPlus5(20) << std::endl;

std::cout << std::endl;

}
```

Each call (lines 1 - 5) gets a callable taking two arguments and returns a callable taking only one argument because the first argument is bound to `2000`. The callable is a function (1), a lambda expression (2), and a predefined function object (line 3). `_1` is a so-called placeholder (line 4) and stands for the missing argument. With lambda expression (line 5), you can directly apply one argument and provide an argument `b` for the missing parameter. From the readability perspective,` std::bind_front` is easier to read than `std::bind` or the lambda expression. If you want to play with the example, use the Compiler Explorer.

## What's next?

In my next post to C++20, I present the extensions of the chrono library: time of day, a calendar, and time zones.

Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Venkata Ramesh Gudpati, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Darshan Mody, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Wolfgang Gärtner,  Louis St-Amour, Stephan Roslen, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Avi Kohn, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, Kai, and Sudhakar Balagurusamy.

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, and Dendi Suhubdy

Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Darshan Mody, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Wolfgang Gärtner,  Louis St-Amour, Stephan Roslen, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Avi Kohn, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, Sudhakar Balagurusamy, lennonli, and Pramod Tikare Muralidhara.

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, and Dendi Suhubdy

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