The unharnessed power of C++ - Lambda functions

The C++ Gurus all over the world have spoken on multiple occassions about how the language has helped shape the world in many ways. OS, Google, Mobile, Wall Street, it’s literally everywhere. Yet, credit to the C++ standards committee, they come up with new features to add every revision/sub-revision. In my opinion, there are a few such features that were borrowed from other programming paradigms that helps C++ stay relevant. In many cases, outperforming the newer languages.

One of the most important and powerful features added in recent times to C++ is Lambda. Lambda function in itself is not a new invention, it has been around in functional programming languages like Lisp for many decades and has been a part of CS Theory discussions for longer.

C++ Lambda:

The lambda function was introduced in C++-11 and enhanced with polymorphic capabilities in C++14. As discussed in my earlier post C++ syntax, these functions can be passed to STL algorithms in C++ to achieve powerful results. They can also be used to perform loose versions of meta programming and store lambda function in std::function.

In theory, a lot of these can be achieved with with defining Functors() or Classes with operator () overloaded.

The below example is taken from C++ High Performance Book

In a nutshell, the lambda function capability enables programmers to pass functions to regular functions, just as easily as a variable is passed.

// Prerequisite 
auto vals = std::vector<int>{1, 3, 2, 5, 4}; 
 
// Look for number three 
auto three = 3; 
auto num_threes = std::count(vals.begin(), vals.end(), three); 
// num_threes is 1 
 
// Count for numbers larger than three 
auto is_above_3 = [](int v){ return v > 3; }; 
auto num_above_3 = std::count_if(vals.begin(), vals.end(), is_above_3);
// num_above_3 is 2 

We pass a variable to run with std::count() and a function called std::count_if() to search with.

We can put an argument or a variable into the lambda function just like an argument in to an equation.

auto num_3 = std::count(vals.begin(), vals.end(), 3); 
auto num_above_3 = std::count_if(vals.begin(), vals.end(), [] (int v) { 
        return v > 3; 
    }
); 

Source:

• C++ High Performance Book