TimelineCpp20CoreLanguage

More and More Utilities in C++20

Today, I present a few utilities for calculating the midpoint of two values, checking if a std::string starts or ends with a substring, and creating callables with std::bind_front. These little utilities may not seem so minor when you need them.

TimelineCpp20CoreLanguage

Let’s start with 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 pointers, they must 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 Compiler Explorer.

midpointLerp

 

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    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 compilers 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 checks 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).

    stringStartsWithEndsWith

     

    The following utility function in C++20 may wonder you.

    std::bind_front

    std::bind_front (Func&& func, Args&& ... args) creates a callable wrapper for a callable func. std::bind_front that 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 Technical Report 1 (TR1). Both can be used as a replacement of std::bind_front. Furthermore, std::bind_front seems like the minor 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.

    bindFront

    If you want to play with the example, use 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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