Question

I’m trying to use libsvm for a certain complex application and, because libsvm is, mostly, a C library, one has to use a custom API function to release memory, after loading certain data. Here is what I mean:

struct svm_model *model;
model = svm_load_model("path to model file");

//do some processing

svm_free_and_destroy_model(&this->model);

And these are the definitions of the libsvm API functions that I used:

struct svm_model *svm_load_model(const char *model_file_name);
void svm_free_and_destroy_model(struct svm_model **model_ptr_ptr);

Although this works just fine, if an exception occurs while I process the model data, then I’ll end up with memory leaks. To prevent this, I wrapped the above code in a class, where I call svm_load_model in the constructor and svm_free_and_destroy_model in the destructor.

Now, since we are in the era of smart pointers, I was thinking to get a bit more creative, and, somehow, declare the model variable as an std::unique_ptr, setting a pointer to svm_free_and_destroy_model as the custom deallocator, but, unfortunately, I’m not able to figure out if such a thing is doable. At the moment, I’m not even able to make it compile and I’m just shooting in the dark. Here is how I think it should work:

std::unique_ptr<struct svm_model *, /* what should I add here? */ > model (svm_load_model("path to model file"), svm_free_and_destroy_model);

Answer 1

The type argument to std::unique_ptr needs to be T, not T *. Use a lambda to call the deleter function.

std::unique_ptr<svm_model, void(*)(svm_model *)> 
  p( svm_load_model( "path_to_model" ), 
     []( svm_model *mdl ) { 
       svm_free_and_detroy_model( &mdl ); 
     } 
   );

Since VS2010 does not implement conversion of stateless lambdas to function pointers, you’ll have to use std::function to get this to work.

std::unique_ptr<svm_model, std::function<void(svm_model*)>>
  p( svm_load_model("path_to_model"), 
     []( svm_model *mdl ) {
       svm_free_and_destroy_model( &mdl );
     }
   );