Thread Safety of shared_ptr

shared_ptr

objects offer the same level of thread safety as built-in types. A

shared_ptr

instance can be "read" (accessed using only const operations) simultaneously by multiple threads. Different

shared_ptr

instances can be "written to" (accessed using mutable operations such as

operator=

or

reset

) simultaneously by multiple threads (even when these instances are copies, and share the same reference count underneath.)

Any other simultaneous accesses result in undefined behavior.

Examples:

shared_ptr<int> p(new int(42));

//--- Example 1 ---

// thread A
shared_ptr<int> p2(p); // reads p

// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe

//--- Example 2 ---

// thread A
p.reset(new int(1912)); // writes p

// thread B
p2.reset(); // OK, writes p2

//--- Example 3 ---

// thread A
p = p3; // reads p3, writes p

// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write

//--- Example 4 ---

// thread A
p3 = p2; // reads p2, writes p3

// thread B
// p2 goes out of scope: undefined, the destructor is considered a "write access"

//--- Example 5 ---

// thread A
p3.reset(new int(1));

// thread B
p3.reset(new int(2)); // undefined, multiple writes