N2587: Minimal Garbage Collection Status API

The goal of this proposal is to provide an API that a program can use to determine if garbage collection or leak detection as described in N2586 is enabled. Reasons this is useful include:

• The concern expressed in the Library Working Group that it may be awkward to detect when garbage collection is enabled on code that relies on behavior that N2586 specifies as undefined in the presence of garbage collection.
• Some programs may wish to behave differently in the presence of garbage collectors or leak detectors. For example, code that uses the "XOR trick" may choose to call declare_reachable when a leak detector is enabled to avoid spurious leaks being reported, but avoid the overhead (likely just the overhead of calling an empty library function) of calling declare_reachable on each list operation during deployment.
• In a similar vein, this can help leak detectors produce more accurate reports for programs that are not suitable for running in a garbage collected environment. Since programs that are not suitable for deployment with garbage collection arguably stand to benefit the most from leak detection, this helps make garbage-collection based technologies benefit important additional classes programs.
• Analogous APIs have proven useful in memory analysis tools, such as Purify's purify_is_running() API.

Library Wording

namespace std {
    enum invalidly_derived_pointer_behavior {
        invalid = 0;
        valid = 1;
        leak_detection = 2;
    };
}

Returns:

valid if all dynamically allocated memory is implicitly declared reachable [Note This guarantess C++03 behavior];
invalid if memory allocated by the global operator new is not declared reachable;
leak_detection in implementation-defined circumstances to indicate that although C++03 behavior is in effect, external tools such as leak detectors are enabled and may produce more accurate reports if declare_reachable declarations are used.

Observations and Issues

• We will come up with shorter names prior to the upcoming meeting, but that is a bicycle shed. Essentially, invalid means (minimal) garbage collection may be enabled, valid means garbage collection is not enabled, and leak_detection means that that garbage collection is not enabled so program behavior is not affected, but garbage detection is.
• The values above are chosen so that leak detection and validity can be considered as bit maps, easing use.
• Since the library allocator is required to use operator new, this applies to the library allocator as well.