Lambda expressions in C++
The lambda expression was introduced in C++11 and convenient way of writing
Syntax
[ capture_clause ] (parameters) -> return_type
{
lambda_body
}
- capture_clause: Specifies which variables are
captured accessed from the surrounding scope []is the empty clause, which means that the lambda expression accesses no variables in the enclosing scope.[&]means all variables that you refer to are captured by reference.[=]means they are captured by value, e.g.,[a, &b]capturesaby value andbby reference. - parameters: These are optional to the function, just like regular parameters in C++ functions.
- return_type: Optional return type, e.g.,
int. - lambda_body: Body of the lambda function.
The
capture_clauseis also known as lambda-introducer. Learn more in the offical docs.
Code
In this example, we sort a vector descendingly. By default, the sort() function sorts ascendingly. However, we can pass a lambda expression as the third argument to the sort() function to have our own custom way of comparing vector elements. The lambda expression has an empty capture clause that takes two parameters (a and b) and returns a boolean.
#include <iostream>#include <vector>#include <stdlib.h> /* srand, rand */#include <algorithm>using namespace std;// Helper function to print a vectorvoid printVector(vector<int> V) {for (int i=0; i < V.size(); i++)cout << V[i] << " ";cout << endl;}int main() {vector<int> V {3, 2, 5, 7, 1, 4, 6};cout << "Before:\t";printVector(V);// The third argument to the sort function is a lambda expression// that has an empty capture clause, has two parameters a and b,// and returns a boolean. The lambda body is just a > b in order// to sort the values descendingly.sort(V.begin(), V.end(), [](const int& a, const int& b) -> bool{return a > b;});cout << "After:\t";printVector(V);}
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