More and More Utilities in C++20

This is a cross-post from www.ModernesCpp.com.

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.

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