c++ [&] operator [duplicate]

Question

hello i have found this code on windows documentation

but i don't get what means

[&]

just please can you clear me what it should do ??

it is not c++ standard true??

This is the code:

void parallel_matrix_multiply(double** m1, double** m2, double** result, size_t size)
{
   parallel_for (size_t(0), size, [&](size_t i)
   {
      for (size_t j = 0; j < size; j++)
      {
         double temp = 0;
         for (int k = 0; k < size; k++)
         {
            temp += m1[i][k] * m2[k][j];
         }
         result[i][j] = temp;
      }
   });
}

Answer 1

It introduces a lambda expression. The contents of the square brackets indicate what is to be captured inside the lambda. Having only a & in there means that everything that is mentioned inside the lambda and can be found outside of its scope is captured by reference.

Example:

int a = 0;
auto l = [&]() { 
  ++a; // a refers to the a outside of this scope through a reference
}
l(); // execute the lambda

Answer 2

It is a C++11 feature and is called the lambda capture clause. In this case, the [&] makes available to the lambda function all of the arguments to the parallel_matrix_multiply() function by reference.

See lambda functions for more information.