Using unique_ptr with gsl_vector - c++

Question

One of my favorite aspects of unique_ptr is the automatic memory management it provides. I would like to use unique_ptr with something like a GSL vector.
However, gsl_vector has its own mechanism for freeing memory (gsl_vector_free). Is there a way to force the unique pointer to use GSL's vector freeing method? Valgrind (correctly) reports a mismatched use of malloc/delete when creating a unique_ptr<gsl_vector> below:
#include <memory>
#include <gsl/gsl_vector.h>
void mem_gsl() {
gsl_vector *vec;
vec = gsl_vector_calloc(2);
gsl_vector_set(vec, 0, 2.0);
printf("%f\n", gsl_vector_get(vec, 0));
gsl_vector_free(vec); // Without this, there is a memory leak.
}
void mem_unique_ptr() {
std::unique_ptr<gsl_vector> vec;
vec.reset(gsl_vector_calloc(2));
// Using .get() each time I want to use the gsl_vector seems like overhead.
gsl_vector_set(vec.get(), 0, 2.0);
printf("%f\n", gsl_vector_get(vec.get(), 0));
// Valgrind correctly reports a mismatched use of delete and free.
}
int main(int argc, char *argv[]) {
mem_gsl();
mem_unique_ptr();
return 0;
}
Additionally, having to call get() each time I want to access a vector element seems rather tedious. Is there a way around this?

Answer 1

You can do it by providing std::unique_ptr with a custom deleter. Something like this would probably do:
auto del = [](gsl_vector* p) { gsl_vector_free(p); };
std::unique_ptr<gsl_vector, decltype(del)> vec(gsl_vector_calloc(2), del);

Answer 2

Having a make_unique_gsl_vector amd a custom deleter:
#include <memory>
// Fake gsl
typedef int gsl_vector;
gsl_vector* gsl_vector_calloc(std::size_t) { return 0; }
void gsl_vector_free(gsl_vector*) {}
// Deleter
struct gsl_vector_deleter {
void operator () (gsl_vector* p) {
gsl_vector_free(p);
}
};
// Unique Pointer
typedef std::unique_ptr<gsl_vector, gsl_vector_deleter> unique_gsl_vector;
unique_gsl_vector make_unique_gsl_vector() {
return unique_gsl_vector(gsl_vector_calloc(2));
}
int main() {
make_unique_gsl_vector();
return 0;
}

Answer 3

To avoid the .get() calls and other boilerplate associated with wrapping a gsl_vector in a unique_ptr, you could create a small RAII wrapper.
namespace gsl {
class vector
{
std::unique_ptr<gsl_vector, decltype(&gsl_vector_free)> v_;
public:
explicit vector(std::size_t num)
: v_(gsl_vector_calloc(num), gsl_vector_free)
{}
double operator[](std::size_t idx) const
{
return gsl_vector_get(v_.get(), idx);
}
void set(std::size_t idx, double value)
{
gsl_vector_set(v_.get(), idx, value);
}
};
}
This not only avoids boilerplate, but also allows you to easily extend functionality, say for instance, adding a constructor that takes a std::initializer_list<double> so you can construct and initialize the vector in a single expression.
Live example