Add C++ header with std::allocator implementation.

Author: TravisWhitaker

README.md

@@ -1,2 +1,42 @@
 # libglaswegian
-Use GHC's Memory Allocator from C
+
+Use GHC's Memory Allocator from C/C++. Yes, I'm serious. You can do this:
+
+```c++
+#include <iostream>
+#include <vector>
+
+#include <glaswegian++.h>
+
+#define LEN 4096
+
+int main()
+{
+    ghc_init();
+    std::vector<double, GlaswegianAllocator<double>> xs;
+    for(int i = 0; i < LEN; i++)
+    {
+        xs.emplace_back(i * i);
+    }
+    double s = 0;
+    for(double i : xs)
+    {
+        s += i;
+    }
+    std::cout << s << std::endl;
+    return 0;
+}
+```
+
+This works by allocating pinned `MutableByteArray#`s, then sticking a
+`StablePtr` into a radix tree with the `MutableByteArray#`'s address as the key.
+This keeps the array alive on the heap until the caller frees the memory (which
+just deletes the `StablePtr` from the tree and does `freeStablePtr`).
+
+A further optimization might be to call the RTS' C functions for doing
+allocations directly, but this library is *shockingly* competitive with other
+memory allocators. I'll add benchmarks soon.
+
+This library is a fun toy I made for some silly benchmarks I wanted to run. It
+is probably only useful for getting your colleagues to shut up about how
+"garbage collectors are slow."

include/glaswegian++.h

@@ -0,0 +1,43 @@
+#include <limits>
+
+extern "C"
+{
+#include <glaswegian.h>
+};
+
+// Call ghc_init before constructing one of these.
+template <typename a>
+struct GlaswegianAllocator
+{
+    typedef a value_type;
+
+    GlaswegianAllocator() = default;
+
+    a* allocate(std::size_t n)
+    {
+        if(n > std::numeric_limits<std::size_t>::max() / sizeof(a))
+        {
+            throw std::bad_alloc();
+        }
+        if(auto p = static_cast<a*>(ghc_alloc(n*sizeof(a))))
+        {
+            return p;
+        }
+        throw std::bad_alloc();
+    }
+    void deallocate(a* p, std::size_t) noexcept
+    {
+        ghc_free(p);
+    }
+};
+
+template <typename a, typename b>
+bool operator==(const GlaswegianAllocator<a>&, const GlaswegianAllocator<b>&)
+{
+    return true;
+}
+template <typename a, typename b>
+bool operator!=(const GlaswegianAllocator<a>&, const GlaswegianAllocator<b>&)
+{
+    return false;
+}

libglaswegian.cabal

@@ -27,6 +27,7 @@ foreign-library glaswegian
                        cbits/ghc_cleanup.c
   include-dirs:        include
   install-includes:    glaswegian.h
+                       glaswegian++.h
   ghc-options:         -O2
                        -threaded
                        -Wall

test/main.cc

@@ -0,0 +1,23 @@
+#include <iostream>
+#include <vector>
+
+#include <glaswegian++.h>
+
+#define LEN 4096
+
+int main()
+{
+    ghc_init();
+    std::vector<double, GlaswegianAllocator<double>> xs;
+    for(int i = 0; i < LEN; i++)
+    {
+        xs.emplace_back(i * i);
+    }
+    double s = 0;
+    for(double i : xs)
+    {
+        s += i;
+    }
+    std::cout << s << std::endl;
+    return 0;
+}