I am reading through Anthony Williams' "C++ Concurrency in Action" and in Chapter 5, which talks about the new multithreading-aware memory model and atomic operations, and he states:
In order to use
std::atomic<UDT>
for some user-definedUDT
, this type must have a trivial copy assignment operator.
As I understand it, this means that we can use std::atomic<UDT>
if the following returns true:
std::is_trivially_copyable<UDT>::value
By this logic, we shouldn't be able to use std::string
as a template argument for std::atomic
and have it work correctly.
However, the following code compiles and runs with expected output:
#include <atomic>
#include <thread>
#include <iostream>
#include <string>
int main()
{
std::atomic<std::string> atomicString;
atomicString.store( "TestString1" );
std::cout << atomicString.load() << std::endl;
atomicString.store( "TestString2" );
std::cout << atomicString.load() << std::endl;
return 0;
}
Is this a case of undefined behaviour which just happens to behave as expected?
Thanks in advance!
The standard does not specify a specialization of std::atomic<std::string>
, so the generic template <typename T> std::atomic<T>
applies. 29.5 [atomics.types.generic] p1 states:
There is a generic class template atomic. The type of the template argument T shall be trivially copyable (3.9).
There is no statement that the implementation must diagnose violations of this requirement. So either (a) your use of std::atomic<std::string>
invokes undefined behavior, or (b) your implementation provides std::atomic<std::string>
as a conforming extension.
Looking at the MSDN page for std::atomic<T>
(http://msdn.microsoft.com/en-us/library/vstudio/hh874651.aspx), it does explicitly mention the requirement that T
be trivially copyable, and it does NOT say anything specific about std::atomic<std::string>
. If it is an extension, it's undocumented. My money is on undefined behavior.
Specifically, 17.6.4.8/1 applies (with thanks to Daniel Krügler for setting me straight):
In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.
std::string
certainly does not meet the std::atomic<T>
requirement that the template parameter T
be trivially copyable, so the standard places no requirements on the implementation. As a quality of implementation issue, note that static_assert(std::is_trivially_copyable<T>::value, "std::atomic<T> requires T to be trivially copyable");
is an easy diagnostic to catch this violation.
2016-04-19 Update: I don't know when the change happened, but VS2015 Update 2 does now diagnose std::atomic<std::string>
:
error C2338: atomic requires T to be trivially copyable.