Lambda function is quite an intuitive concept of introduced in C++11, So there are already tons of articles on lambda function tutorial over the internet. But still, there are some untold things(like IIFE, types of lambda, etc.) left, which nobody talks about. Therefore, here I am to not only show you lambda function in C++ but we'll also cover how it works internally & other aspects of Lambda. Modern C++ /!\: Originally published @ . www.vishalchovatiya.com Title of this article is a bit misleading. Because . It's an expression(precisely unique closure). But I have kept it that way for simplicity. So from now on, I will use lambda function & expression interchangeably. lambda doesn't always synthesize to function pointer What is lambda function? A lambda function is short snippets of code that not worth naming(unnamed, anonymous, disposable, etc. Whatever you can call it), and also not reused. In other words, it's just syntactic sugar. lambda function syntax is defined as: [ capture list ] (parameters) -> return-type  
{   
    method definition
} Usually, . So we don't need to specify a trailing return type explicitly i.e. . compiler evaluates a return type of a lambda function itself -> return-type But in some complex cases, the compiler unable to deduce the return type and we need to specify that. Why we should use a lambda function? C++ includes many useful generic functions like , which can be handy. Unfortunately, they can also be quite cumbersome to use, particularly if the functor you would like to apply is unique to the particular function. Consider the following code for an example: std::for_each { << element << ;
    }
}; { :: < > v = { , , , , }; ::for_each(v.begin(), v.end(), print()); ;
} { struct print void operator () ( element) int cout endl int main ( ) void std vector int 1 2 3 4 5 std return 0 If you use print once, in that specific place, it seems overkill to be writing a whole class just to do something trivial and one-off. However, for this kind of situation inline code would be more suitable & appropriate which can be achieved by lambda function as follows: ::for_each(v.begin(), v.end(), []( element) { << element << ; }); std int cout endl How lambda functions works internally? [&i] ( ) { :: << i; } &m_i;
    anonymous( &i) : m_i(i) {} { :: << i;
    }
}; std cout // is equivalent to { struct anonymous int int inline auto operator () () const std cout The . Finally, the secret revealed. compiler generates unique closure as above for each lambda function Capture list will become a constructor argument in closure, If you capture argument as value then corresponding type data member is created within the closure. Moreover, you can declare variable/object in the lambda function argument, which will become an argument to call operator i.e. . operator() Benefits of using a lambda function Zero cost abstraction. Yes! you read it right. . lambda doesn't cost you performance & as fast as a normal function In addition, code becomes compact, structured & expressive. Learning lambda expression Capture by reference/value { x = , y = ; print = [&] { :: << __PRETTY_FUNCTION__ << << x << << y << :: ;
    };
    print(); ;
} int main () int 100 200 auto // Capturing object by reference std cout " : " " , " std endl return 0 Output: main()::<lambda()> : 100 , 200 In the above example, I have mentioned in capture list. which captures variable & as reference. Similarly, denotes captured by value, which will create data member of the same type within the closure and copy assignment will take place. & x y = Note that, the parameter list is optional, to the lambda expression. you can omit the empty parentheses if you do not pass arguments Lambda capture list The following table shows different use cases for the same: Passing lambda as parameter < Functor> { :: << __PRETTY_FUNCTION__ << :: ;
} { i = ; i++; } { lambda_func = [i = ]() { i++; };
    f(lambda_func); f(g); } template typename void f (Functor functor) std cout std endl /* Or alternatively you can use this
void f(std::function<int(int)> functor)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
} 
*/ int g () static int 0 return int main () auto 0 mutable return // Pass lambda // Pass function Output: Function Type : void f(Functor) [with Functor = main()::<lambda(int)>]
Function Type : void f(Functor) [with Functor = int (*)(int)] You can also pass lambda function as an argument to other function just like a normal function which I have coded above. If you noticed, here I have declared variable i in capture list which will become data member. As a result, every time you call lambda_func, it will be returned and incremented. Capture member variable in lambda or this pointer :
    Example() : m_var( ) {} {
        [=]() { :: << m_var << :: ; }(); } : m_var;
}; {
    Example e;
    e.func();
} { class Example public 10 void func () std cout std endl // IIFE private int int main () pointer can also be captured using , or . In any of these cases, class data members(including ) can be accessed as you do in a normal method. this [this] [=] [&] private If you see the lambda expression line, I have used extra at the end of the lambda function declaration which used to calls it right thereafter declaration. It is called ( ). () IIFE Immediately Invoked Function Expression C++ lambda function types Generic lambda l = []( a, b, c) {}; < }
}; const auto auto auto auto // is equivalent to { struct anonymous template , , > ()( , , ) { class T0 class T1 class T2 auto operator T0 a T1 b T2 c const Generic lambda introduced in C++14 which can captures parameters with specifier. auto Variadic generic lambda {} < First, ... Rest> { :: << first << :: ;
    print(args...);
} { variadic_generic_lambda = []( ... param) {
        print(param...);
    };
    variadic_generic_lambda( , , );
} void print () template typename typename void print ( First &first, Rest &&... args) const std cout std endl int main () auto auto 1 "lol" 1.1 Lambda with variable parameter pack will be useful in many scenarios like debugging, repeated operation with different data input, etc. lambda function mutable Typically, a lambda's function call operator is const-by-value which means lambda requires mutable  keyword if you are capturing anything by-value. []() {} {
    }
}; mutable // is equivalent to { struct anonymous auto operator () () // call operator We have already seen an example of this above. I hope you noticed it. Lambda as a function pointer { funcPtr = +[] {}; ( ::is_same< (funcPtr), (*)()>::value);
} # include <iostream> # include <type_traits> int main () auto static_assert std decltype void You can force the compiler to generate lambda as a function pointer rather than closure by adding infront of it as above. + Higher-order returning lambda functions less_than = []( x) { [x]( y) { y < x;
    };
}; { less_than_five = less_than( ); :: << less_than_five( ) << :: ; :: << less_than_five( ) << :: ; ;
} const auto auto return auto return int main ( ) void auto 5 std cout 3 std endl std cout 10 std endl return 0 Going a bit further, lambda function can also return another lambda function. This will open the doors of endless possibility for customization, code expressiveness & compactibility(BTW, there is no word like this) of code. lambda expression constexpr Since C++17, a lambda expression can be declared as . constexpr sum = []( &a, &b) { a + b; }; answer = sum( , ); constexpr auto const auto const auto return /*
    is equivalent to
    constexpr struct anonymous
    {
        template <class T1, class T2>
        constexpr auto operator()(T1 a, T2 b) const
        {
            return a + b;
        }
    };
*/ constexpr int 10 10 Even if you don't specify , the function call operator will be anyway, if it happens to satisfy all . constexpr constexpr constexpr function requirements Closing words I hope you enjoyed this article. I have tried to cover most of the intricacies around lambda with a couple of simple & small examples. You should use lambda wherever it strikes in your mind considering code expressiveness & easy maintainability like you can use it in custom deleters for smart pointers & with most of the STL algorithms.

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