Following my series from less known features of Modern C++.
Lambdas
Lambdas are anonymous functions which are un-named functions as they are mostly needed for High Order Functions and for small purposes where they don't need much importance to be named. Lambdas were introduced in C++11 and heavily improved in the new standards.
Syntax
[capture list](parameter list) -> return type (optional) {
function body
}
- Capture List: Specifies how the value will be captured in the scope.
- Parameter List: Parameters required for the function
- Return Type (Optional) : The return type of the function.
- Function Body: Body of the function.
Basic Examples
#include <iostream>
int main() {
int add = [](int a){ return 2 + 3; }(); // using lambda and then storing the value
[](){ std::cout<< "Caliing like this also works" << std::endl; }();
auto callback = [](auto a, auto b) {
a == b;
}; // using a label and then using it for various purposes
}
Capture List
A lambda can introduce new objects in its body and can also access or capture objects from the surrounding scope. A lambda begins with the capture clause which specifies the objects that are captured and whether the capture is by value or by reference. Objects that have the ampersand (&) prefix are accessed by reference and objects that do not have it are accessed by value.
An empty capture list [] means that there are no variables captured. If the user wants all variables to be captured by reference then one should use [&] or pass by value then use [=]. If one wants some variable to be captured by reference or value then they should do [foo, &var].
The following are valid:
[&var, foo] // var is captured by reference and foo is passed by value.
[&, factor] // only factor is captured by value here.
[=, &total] // only total is passed by reference.
[] // no objects are passed.
Parameter List
Like functions, parameters are optional for lambdas.
#include <iostream>
int main() {
auto add = [](int a, int b) -> int {
return a + b;
};
auto print = []() {
std::cout << "Printing from lambda." << std::endl;
};
std::cout << add(5, 5) << std::endl;
std::cout << print() << std::endl;
}
In this example we have add() lambda which takes parameter and print() lambda which doesn't take.
Since C++ 14, we can now use auto for type deceleration which can be helpful for combating more problems.
#include <iostream>
int main() {
auto add = [](auto a, auto b) {
return a + b;
};
std::cout << add(5, 5) << std::endl; // for int
std::cout << add(24.567, 456.7) << std::endl; // for float
std::cout << add("generic", "lambdas") << std::endl; // for std::string
}
mutable Specifier
mutable allows objects captured by value to be modified and use it's non-constant methods/functions in the specific lambda function.
#include <iostream>
int main() {
int var = 5;
auto change = [var]() mutable { var = 10; std::cout << var << std::endl; };
change();
std::cout << var << std::endl; // var is 5
}
Examples
Nested Lambdas
A simple program which takes input and then determines if they are negative or not.
#include <iostream>
int main() {
auto is_negative = [](int x) { return [](int y) { return y + 2; }((5) + x) < 0; }(7);
std::cout << is_negative;
}
High Order Functions
A simple program which takes a function to sort an array in the reverse order.
#include <iostream>
#include <algorithm>
#include <array>
int main() {
std::array<int, 9> arr = {4, 6, 7, 5, 8, 0, 2, 3, 7 };
std::sort(begin(arr), end(arr), [](int a, int b){ return a < b;});
for (auto& i : arr) {
std::cout << i << " ";
}
}
Lambdas with Templates
A program which uses templates and lambdas together.
#include <iostream>
#include <algorithm>
#include <array>
template<typename T>
void multi_by_two(std::array<T, 9>& arr) {
std::for_each(arr.begin(), arr.end(), [](T& a){a *= 2;});
}
int main() {
std::array<int, 9> arr = {4, 6, 7, 5, 8, 0, 2, 3, 7 };
add_by_two(arr);
for (auto& i : arr) {
std::cout << i << " ";
}
}
Code
All the code used in this article is available in this repo.
Delta456
/
modern_cpp_series
Code for my Modern C++ Articles
Modern C++ Series
Code for my Modern C++ Article Series.
Articles
If someone has any doubt or something is misleading then do inform me by commenting below.
Top comments (2)
[=, &total] please explain what the = does on this line?
It passes all objects by value.