We present Offload, a programming model for offloading parts of a C++ application to run on accelerator cores in a heterogeneous multicore system. Code to be offloaded is enclosed in an offload scope; all functions called indirectly from an offload scope are compiled for the accelerator cores. Data defined inside/outside an offload scope resides in accelerator/host memory respectively, and code to move data between memory spaces is generated automatically by the compiler. This is achieved by distinguishing between host and accelerator pointers at the type level, and compiling multiple versions of functions based on pointer parameter configurations using automatic call-graph duplication. We discuss solutions to several challenging issues related to call-graph duplication, and present an implementation of Offload for the Cell BE processor, evaluated using a number of benchmarks.