've got an API like this, but for many cases I don't have to use it. For most engine-provided data, I can instead do:
perObjectVSData = CBuffer::Create( sizeof(StructThatIPromiseMatchesMyHLSL) );
And for debugging:
CBufferInfo* cb = model->Effect()->FindCBuffer("cbPerObjectVS");
ASSERT( cb->SizeInBytes() == sizeof(StructThatIPromiseMatchesMyHLSL) );
ASSERT( OFFSETOF(StructThatIPromiseMatchesMyHLSL::foo) == cb->OffsetOf("foo") );
In my engine setup, all C structures that are required to match against HLSL cbuffers are compile-time forced provide a decl function for matching the alignments.
I have a templated subclass of the buffer object that turns CBuffer::Create( sizeof(StructThatIPromiseMatchesMyHLSL) ); into TCBuffer<structthatipromisematchesmyhlsl>, which when compiled under paranoia flags performs validation.
FWIW, I've taken the same approach to declaring structures in this way for vertex stream formats and pixel shader outputs.
The pixel shader output being defined in said way, allows my shader compiler to split a pixel shader with say, 7 float4 outputs into 2 seperate shaders for platforms that only support 4 targets under MRT.
The vertex stream format decl was helpful in getting hassle-free instancing up and running. The Transform CBuffer struct can be easily treated as a secondary vertex stream for DrawInstanced calls.</structthatipromisematchesmyhlsl>