将 unique_ptr 与 GError 的自定义删除器一起使用

Question

using unique_ptr with custom deleter for c functions that return a ptr is pretty straight forward, but how would i use it for functions that take ptr of ptr as parameter (like GError)

I have faced this in couple of cases but but did not find a straightforward way to do this. am i missing something?

using Element = std::unique_ptr<GstElement, void (*)(GstElement*)>;
using Error = std::unique_ptr<GError, void (*)(GError*)>;
...
GError* plain_err;
Element pipeline(gst_parse_launch(str, &plain_err), object_unref); // ok

Error uniq_err {nullptr, error_free};
Element pipeline(gst_parse_launch(str, &(uniq_err.get())), object_unref); // error: lvalue required as unary ‘&’ operand

The only way I could think of is by assigning it later

GError* plain_err;
Element pipeline(gst_parse_launch(str, &plain_err), object_unref);
Error uniq_err {plain_err, error_free};
plain_error = nullptr;

Answer 1

This part of your code:

GError* plain_err;
Element pipeline(gst_parse_launch(str, &plain_err), object_unref);

suggest that gst_parse_launch() allocates GError* and returns it. In this case, you cannot use unique_ptr your way. Because unique_ptr just manages lifetime of a pointer.

You can try something like this for managing your pointer using unique_ptr :

struct ErrorRelease {
    void operator()(GError* ptr) {
        // release pointer
    }
};
using Error = std::unique_ptr<GError, ErrorRelease>;

GError* plain_err;
Element pipeline(gst_parse_launch(str, &plain_err), object_unref);

Error uniq_err(plain_err);

After this, you just must remember not to free plain_err manually because it is managed by your unique_ptr . It is OK to assign another value to it, just don't release it manually.

Answer 2

I found returning a tuple from a lambda to be marginally better,

auto [pipeline, err] = [&]() noexcept {
    GError* err = nullptr;
    auto* pipeline = gst_parse_launch(str, &err);
    return std::tuple{Element{pipeline, object_unref}, Error{err, error_free}};
}();