Why isn't this unused variable optimised away?

Question

I played around with Godbolt\'s CompilerExplorer. I wanted to see how good certain optimizations are. My minimum working example is:

#include 

int         


        

Answer 1

std::vector is a fairly complicated class that involves dynamic allocation. While clang++ is sometimes able to elide heap allocations, it is a fairly tricky optimization and you should not rely on it. Example:

int foo() {
    int* p = new int{5};
    return *p;
}

foo():                                # @foo()
        mov     eax, 5
        ret

---

As an example, using std::array (which does not dynamically allocate) produces fully-inlined code:

#include 

int foo() {
    std::array v{1, 2, 3, 4, 5};
    return v[4];
}

foo():                                # @foo()
        mov     eax, 5
        ret

---

As Marc Glisse noted in the other answer's comments, this is what the Standard says in [expr.new] #10:

An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]). When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression. The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended: [...]