In my example code below, I'd like to know when two calls to log_cref_address
will reliably print the same address.
#include <iostream>
#include <thread>
#include <functional>
using namespace std;
void log_cref_address(const int& t) {
cout << addressof(t) << ' ';
}
template <int i>
void foo() {
log_cref_address(i); // different if foo called from different threads
thread([] { log_cref_address(i); }).join(); // same if already in thread
thread(log_cref_address, i).join(); // same if already in thread
cout << endl;
}
int main() {
// first three calls print identical addresses
cout << "foo<0>: "; foo<0>();
cout << "foo<0>: "; foo<0>();
cout << "foo<1>: "; foo<1>();
cout << endl;
// last two from thread yields different addresses from the first three
cout << "lambda: "; thread([] { foo<0>(); }).join();
cout << "bind(): "; thread(bind(foo<0>)).join();
return 0;
}
On my machine, main
prints
foo<0>: 0x7fff7cf5507c 0x7fa0585b5e1c 0x196fc28
foo<0>: 0x7fff7cf5507c 0x7fa0585b5e1c 0x196fc28
foo<1>: 0x7fff7cf5507c 0x7fa0585b5e1c 0x196fc28
lambda: 0x7fa0585b5dcc 0x7fa057db4e1c 0x7fa0500008c8
bind(): 0x7fa0585b5d1c 0x7fa057db4e1c 0x7fa0500008c8
From many outputs like this, I've observed that main
behaves as follows:
foo
print identical addresses.foo
(from threads) print addresses not printed by the first three calls.foo
, log_cref_address
prints the same address if and only if called from a sub-thread.Which (if any) of these behaviors are guaranteed by the C++ standard, on any machine?
None of them. The standard doesn't guarantee the address of a temporary variable.