Form of Lambda Expression

[capture list](parameter list) mutable noexcept -> return-type {body}

Internally, compiler generates a class with function call operator().

Capture List

Specifies variables in the enclosing scope which need to be captured by value or by reference

  • [ ] Empty capture list means that lambda expression accesses no variables in the enclosing scope.
  • [=] means all the variables in the enclosing scope are captured by value.
  • [&] means all the variables in the enclosing scope are captured by reference.
int i=100;
auto lambda = [&i]() mutable -> void { i = i * i; };
lambda();
cout << i << endl; // Prints 10000

int i=100;
auto lambda = [i]() mutable -> void { i = i * i; };
lambda();
cout << i << endl; // Prints 100

Parameter List

Specifies input parameters for lambda.

    int i=20;
    auto lambda = [](int i) -> int  { return i * i; } ;
    int square = lambda(i);
    cout << square << endl; // Prints 40

Mutable Specification

By default lambda’s function call operator() is const-by-value. Mutable specification allows lambda to modify variables which are captured by reference.

Exception Specification

noexcept exception specification to indicate that the lambda expression does not throw any exceptions.

Return type

Compiler automatically deduces return type if a trailing-return-type is not explicitly specified.

Example Usage with STL Algorithms

    vector<int> numbers {1, 2, 3};    
    for_each(numbers.begin(), numbers.end(),
        [](int element) { cout << element << " ";}
    );

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