Create object after ID is allocated in std::unordered_map - c++

Question

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?
EDIT:
I've played around with the code and used a solution herein to demonstrate:
void add(int id, Foo *f)
{
auto iden = id;
auto foo = f;
fm[iden] = foo;
}
void add(Foo *f, int id)
{
auto iden = id;
auto foo = f;
fm[iden] = foo;
}
void add()
{
int id = allocateID();
fm[id] = new Foo();
}
In the first add() function, the output remains the same. This doesn't solve the problem.
In the second add() function, the output is different, instead producing:
Allocating ID
Foo created!
And in the third function, it has been modified with one of the solutions: this function also produces the desired output. The problem with the third function is that it's highly unlikely there would be an empty add() function realistically. The only culpable way of solving this issue is to pass the object first AND THEN the id. This is probably because the arguments are read from right to left, regardless of the entry requirements because they are created after the arguments are read anyway.

Answer 1

You can't rely on order of evaluation of function arguments. Try this:
void add()
{
int temp = allocateID();
fm.emplace(temp, new Foo());
}

Answer 2

Unless you are absolutely sure how the compiler will interpret your code, you don't want to rely on it and let it tell you what you wanted to do. Since you are having problems you should force the compiler to create the id first as such:
void add()
{
int id = allocateID();
fm[id] = new Foo();
}
This will fix your problems.