How can I overload the new operator for multiple heaps?

Question

In an embedded system with two separate RAM memory areas, I have two distinct heaps (one is a custom implementation from FreeRTOS in the lower memory area, another is the heap generated by GCC in the upper memory area) and I would like to be able to choose which heap new uses.

Answer 1

You could provide an operator new overload that accepts a second argument telling it which memory area to allocate memory from. You can give arguments to operator new by putting them in parenthesis before the type in your new-expression. This is usually used to new an object into already-allocated storage (since that's the overload provided by the standard library), but anything can be passed there and it will be passed on to operator new.

enum MemoryArea {
    LOWER,
    UPPER
};

void* operator new(std::size_t sz, MemoryArea seg) {
    if (seg == LOWER) {
        return allocateMemoryInLowerMemoryArea(sz);
    } else {
        return allocateMemoryInUpperMemoryArea(sz);
    }
}

void operator delete(void* p) {
    if (pointerIsInLowerMemoryArea(p)) {
        freeMemoryFromLowerMemoryArea(p);
    } else {
        freeMemoryFromUpperMemoryArea(p);
    }
}

int main() {
    Foo* p = new (LOWER) Foo;
    Foo* b = new (UPPER) Foo;
    delete p;
    delete b;
}