Modern C++ has a great deal of features and functions designed to make the process of writing apps easier and less labor-intensive. In C++, STL Standard Template Library has many algorithms for some operations like searching, counting, and manipulation of ranges and their elements. C++ has a new feature that you can sort with std::sort Parallel STL Algorithm. C++ has function objects for performing comparisons and they can be used to sort data by std::sort Parallel STL Algorithm with an appropriate C++ Dev Tool that has a C++ compiler.
Table of Contents
What is the std::sort algorithm in C++?
The std::sort Parallel STL algorithm sorts the elements in the range from the first member to the last member in non-descending order.
The syntax for the std::sort algorithm can be described as below,
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void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, <function object> ); // since C++14 |
As an ExecutionPolicy std::execution::seq is used for the sequential execution, std::execution::par is used for the parallel execution . In general, there are 3 ExecutionPolicy objects you can use here,
std::execution::seqstd::execution::parstd::execution::par_unseq
What are the standard library compare function objects in C++?
The standard library compare function objects are used compare data types to be used in other algorithms like sorting algorithms. The standard library compare function objects are included in C++ <functional> library header. Thus, if you want to use them you should add this <functional> library header to your code.
std::greater
std::greater is a function object for performing comparisons from higher to lower values. Unless specialized, std::greater function object invokes operator > on type T. Here is the Syntax;
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template< class T = void > struct greater |
For example we can use it to sort a vector as below,
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std::vector<int> vec(100); std::sort( vec.begin(), vec.end(), std::greater<int>() ); |
std::less
std::less is a function object for performing comparisons from lower to higher values. Unless specialized, std::less function object invokes operator < on type T. Here is the Syntax;
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template< class T = void > struct less; |
For example we can use it to sort a vector as below,
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std::vector<int> vec(100); std::sort( vec.begin(), vec.end(), std::less<int>() ); |
std::greater_equal
std::greater_equal is a function object for performing comparisons. Unless specialized, std::greater_equal function object invokes operator >= on type T. Here is the Syntax;
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template< class T = void > struct greater_equal; |
std::less_equal
std::less_equal is a function object for performing comparisons. Unless specialized, std::less_equal function object invokes operator <= on type T.
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template< class T = void > struct less_equal; |
How do I sort with the standard library compare function objects in C++?
Sometimes we require very specific sorting techniques when we sort members of data in range. In this case we can use function objects.
Here is the Syntax for the std::sort method with a function object,
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void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, <function object> ); // since C++14 |
Let’s assume that we want to use std::greater function object, sort will be as below,
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std::vector<int> vec(100); std::sort( vec.begin(), vec.end(), std::greater<int>() ); |
Is there a full example of how to sort with the standard library compare function objects in C++?
Now, let’s see how we can sort a lot of vector elements with function objects, Here is a full C++ example,
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#include <iostream> #include <vector> #include <algorithm> #include <ctime> int main() { std::vector<int> vec(100000); // fill vector with random numbers std::srand(unsigned(std::time(nullptr))); std::generate(vec.begin(), vec.end(), std::rand); // sort vector std::sort( vec.begin(), vec.end(), std::greater<int>() ); // print first 100 and last 100 members to check if sorted well for(auto it = vec.begin(); it<vec.begin()+100; ++it) std::cout << *it << ','; std::cout << '\n'; for(auto it = vec.end()-100; it<vec.end(); ++it) std::cout << *it << ','; std::cout << '\n'; system("pause"); return 0; } |
Note that, in C++, if you are using arrays, std::array is on the stack, in other words it has less memory limits than vectors.
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