In C++ 11, a lambda expression is a convenient way of defining an anonymous function object right at the location where it is invoked or passed as an argument to a function.

basic usage:

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[=] (int x) mutable throw() -> int
{
  // function
  int n = x + y;
  return n;
}

  • [=]：lambda-introducer, or capture clause
    every lambda function start with lambda-introducer and can not be omitted. Besides of the working as a keyword for lambda expression, it can also denote how to fetch variables into lambda function.
    You can use the default capture mode (capture-default in the Standard syntax) to indicate how to capture any outside variables that are referenced in the lambda: [&] means all variables that you refer to are captured by reference, and [=] means they are captured by value. You can use a default capture mode, and then specify the opposite mode explicitly for specific variables.
    For example, if a lambda body accesses the external variable total by reference and the external variable factor by value, then the following capture clauses are equivalent:

    []: default
    [=]: set default capture mode to capture by value.
    [&]: set default capture mode to capture by reference.
    [x, &y]: x is capture by value, y is capture by reference.
    [=, &y]: every variable is captured by value, except y is captured by reference.
    [&, x]: every variable is captured by reference, except x is captured by value.

use lambda with STL function:
example(with out lambda):

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bool condition(int value) {
  return (value > 5);
}
int main() {
  vector<int> numbers { 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45, 50 };
  // check how many elements meet the condition
  auto count = count_if(numbers.begin(), numbers.end(), condition);
  cout << "Count: " << count << endl;
}


vector<int> numbers { 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45, 50 };
  // use lambda expression to replace the condition function
  auto count = count_if(numbers.begin(), numbers.end(), [](int x) { return (x > 5); });
  cout << "Count: " << count << endl;

reference:
https://msdn.microsoft.com/en-us/library/dd293608.aspx
https://blog.gtwang.org/programming/lambda-expression-in-c11/