C++ – “accummulate_if”

c++stl

Is there a function similar to accummulate() but provides a unary pre-condition to filter the linear container when performing the operation? I search for accummulate_if but there isn't any. Thanks!

update:
Thanks for all the kind answers. I end up doing it this way:

std::for_each(v.begin(), v.end(), [&](int x){if (Pred) sum += x;});

Best Solution

Must you really use an algorithm? Something as simple as below won't do?

for (const auto& v: V)  if(pred(v)) sum+=v;

Sam's idea is also good. But I would do it with lambda:

 sum = accumulate(
     V.begin(), V.end(), 0, 
     [](int a, int b){return pred(b)? a+b: a;}
 );

Placement new allows you to construct an object in memory that's already allocated.

You may want to do this for optimization when you need to construct multiple instances of an object, and it is faster not to re-allocate memory each time you need a new instance. Instead, it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects, even though you don't want to use all of it at once.

DevX gives a good example:

Standard C++ also supports placement new operator, which constructs an object on a pre-allocated buffer. This is useful when building a memory pool, a garbage collector or simply when performance and exception safety are paramount (there's no danger of allocation failure since the memory has already been allocated, and constructing an object on a pre-allocated buffer takes less time):

char *buf  = new char[sizeof(string)]; // pre-allocated buffer
string *p = new (buf) string("hi");    // placement new
string *q = new string("hi");          // ordinary heap allocation

You may also want to be sure there can be no allocation failure at a certain part of critical code (for instance, in code executed by a pacemaker). In that case you would want to allocate memory earlier, then use placement new within the critical section.

Deallocation in placement new

You should not deallocate every object that is using the memory buffer. Instead you should delete[] only the original buffer. You would have to then call the destructors of your classes manually. For a good suggestion on this, please see Stroustrup's FAQ on: Is there a "placement delete"?

Best Solution

Related Solutions

C++ – Do I need to protect read access to an STL container in a multithreading environment

C++ – What uses are there for “placement new”

Placement new allows you to construct an object in memory that's already allocated.

Deallocation in placement new

Related Question