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// Occasionally, it is advantageous to avoid initializing the individual
// elements of a device_vector. For example, the default behavior of
// zero-initializing numeric data may introduce undesirable overhead.
// This example demonstrates how to avoid default construction of a
// device_vector's data by using a custom allocator.
#include <thrust/device_allocator.h>
#include <thrust/device_vector.h>
#include <thrust/logical.h>
#include <thrust/functional.h>
#include <cassert>
// uninitialized_allocator is an allocator which
// derives from device_allocator and which has a
// no-op construct member function
template<typename T>
struct uninitialized_allocator
: thrust::device_allocator<T>
{
// the default generated constructors and destructors are implicitly
// marked __host__ __device__, but the current Thrust device_allocator
// can only be constructed and destroyed on the host; therefore, we
// define these as host only
__host__
uninitialized_allocator() {}
__host__
uninitialized_allocator(const uninitialized_allocator & other)
: thrust::device_allocator<T>(other) {}
__host__
~uninitialized_allocator() {}
#if THRUST_CPP_DIALECT >= 2011
uninitialized_allocator & operator=(const uninitialized_allocator &) = default;
#endif
// for correctness, you should also redefine rebind when you inherit
// from an allocator type; this way, if the allocator is rebound somewhere,
// it's going to be rebound to the correct type - and not to its base
// type for U
template<typename U>
struct rebind
{
typedef uninitialized_allocator<U> other;
};
// note that construct is annotated as
// a __host__ __device__ function
__host__ __device__
void construct(T *)
{
// no-op
}
};
// to make a device_vector which does not initialize its elements,
// use uninitialized_allocator as the 2nd template parameter
typedef thrust::device_vector<float, uninitialized_allocator<float> > uninitialized_vector;
int main()
{
uninitialized_vector vec(10);
// the initial value of vec's 10 elements is undefined
// resize without default value does not initialize elements
vec.resize(20);
// resize with default value does initialize elements
vec.resize(30, 13);
// the value of elements [0,20) is still undefined
// but the value of elements [20,30) is 13:
using namespace thrust::placeholders;
assert(thrust::all_of(vec.begin() + 20, vec.end(), _1 == 13));
return 0;
}
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