/cstruct_out switch

This switch modifies the C definition of a COM interface which returns structures to match the ABI a C++ implementer would provide.

midl /cstruct_out

Switch Options

This switch has no parameters.


Some interface definitions (notably those in d3d12.idl) contain __stdcall methods that return structures. The C and C++ ABIs from MSVC differ in how they implement such functions:

  • C treats them as plain functions that take a hidden this pointer as the first parameter. The complier applies a small struct optimization that allows structs smaller than 8 bytes (or larger if all values are floating point) to be returned in registers. Only larger structures are promoted to use a hidden parameter and caller-allocated return value.
  • C++ treats them as member functions. The compiler always does so by inserting a hidden parameter (a pointer to a caller-allocated return value) as the second parameter, after the this pointer. It also returns that same pointer as its return value.

This switch forces the C definition of interfaces in the resulting header to assume that the implementer was using C++, and that the C code should instead explicitly use the C++ ABI. This implies that the function includes a hidden parameter for the return value pointer and returns that pointer instead of the structure directly.

See also

General MIDL Command-line Syntax