I am currently working on the documentation of the Windows Media Format SDK, and I was confronted with some API's which accept variable sized structs in the DRM Client Extended API. Writing clean memory allocation code for variable structs can be a painful endeavour and I am a lazy lazy man. For my own purposes I developed the a data structure which simplifies usage of data structures with variable sized fields when the size is a constant known at compile-time.

The following code demonstrates how you can write a simple template wrapper around an arbitrary variable sized struct (where the last field of the struct is variable sized).

// Pre-declaration
typedef unsigned int DWORD;
typedef unsigned char BYTE;

// This is an actual struct from the WMF SDK
struct WMDRM_IMPORT_CONTENT_KEY
{
    DWORD dwVersion;
    DWORD cbStructSize;
    DWORD dwIVKeyType;
    DWORD cbIVKey;
    DWORD dwContentKeyType;
    DWORD cbContentKey;
    BYTE rgbKeyData[ 1 ];
};

// This is a somewhat generic template wrapper around a variable sized struct.
template<typename Struct, int FieldSize_N> 
struct VarStruct 
{   
    Struct* operator->() { return &(data_union.as_struct); }    
    static const int field_size = FieldSize_N;
    static const int struct_size = sizeof(Struct) + FieldSize_N - 1;

private:
    
    union
    {
        Struct as_struct;
        BYTE as_array[struct_size];
    } 
    data_union;
};

// Here is some mundane usage. 
int main()
{
    VarStruct<WMDRM_IMPORT_CONTENT_KEY, 42> vs;
    for (int i=0; i < 42; ++i)
    {
        vs->rgbKeyData[i] = i;
    }
}

I hope this code is self-explanatory, and can come in useful. Feel free to fire any questions my way.

The contents of this post are licensed under the Creative Commons Attribution-NonCommercial-NoDerivs license. The opinions in this post are my own and are not neccessarily the views of my employer, Microsoft.