Create object after ID is allocated in std::unordered_map

I have an std::unordered_map which stores an integer with an object. Here's some code for you to understand:

#include <iostream>
#include <unordered_map>
#include <cstdlib>
using namespace std;

class Foo
{
public:
    Foo()
    {
        cout << "Foo created!" << endl;
    }

    ~Foo()
    {

    }
};

typedef std::unordered_map<int, Foo*> FooMap;
FooMap fm;

int allocateID()
{
    cout << "Allocating ID" << endl;
    return rand() % 100;
}

void add()
{
    fm.emplace(allocateID(), new Foo());
}

int main()
{
    srand(time(NULL));

    add();

    return 0;
}

Output:

Foo created!
Allocating ID

The problem here is that the object Foo is created before an ID is allocated! I tried adding a mutex lock on allocateID; it didn't work because allocateID() runs AFTER the object is created.

How can I modify this program such that Foo is created AFTER the ID is allocated?