Lambda Expressions

C++11 lambda expressions make the oneAPI Threading Building Blocks (oneTBB) parallel_for much easier to use. A lambda expression lets the compiler do the tedious work of creating a function object.

Below is the example from the previous section, rewritten with a lambda expression. The lambda expression, replaces both the declaration and construction of function object ApplyFoo in the example of the previous section.

#include "oneapi/tbb.h"


using namespace oneapi::tbb;


void ParallelApplyFoo( float* a, size_t n ) {
   parallel_for( blocked_range<size_t>(0,n),
      [=](const blocked_range<size_t>& r) {
                      for(size_t i=r.begin(); i!=r.end(); ++i)
                          Foo(a[i]);
                  }
    );
}

The [=] introduces the lambda expression. The expression creates a function object very similar to ApplyFoo. When local variables like a and n are declared outside the lambda expression, but used inside it, they are “captured” as fields inside the function object. The [=] specifies that capture is by value. Writing [&] instead would capture the values by reference. After the [=] is the parameter list and definition for the operator() of the generated function object. The compiler documentation says more about lambda expressions and other implemented C++11 features. It is worth reading more complete descriptions of lambda expressions than can fit here, because lambda expressions are a powerful feature for using template libraries in general.

C++11 support is off by default in the compiler. The following table shows the option for turning it on.

Environment	Intel® C++ Compiler Classic	Intel® oneAPI DPC++/C++ Compiler
Windows* OS systems	icl /Qstd=c++11 foo.cpp	icx /Qstd=c++11 foo.cpp
Linux* OS systems	icc -std=c++11 foo.cpp	icx -std=c++11 foo.cpp

For further compactness, oneTBB has a form of parallel_for expressly for parallel looping over a consecutive range of integers. The expression parallel_for(first,last,step,f) is like writing for(auto i=first;         i<last;       i+=step)f(i) except that each f(i) can be evaluated in parallel if resources permit. The step parameter is optional. Here is the previous example rewritten in the compact form:

#include "oneapi/tbb.h"


using namespace oneapi::tbb;


#pragma warning(disable: 588)


void ParallelApplyFoo(float a[], size_t n) {
    parallel_for(size_t(0), n, [=](size_t i) {Foo(a[i]);});
}

The compact form supports only unidimensional iteration spaces of integers and the automatic chunking feature detailed on the following section.