CODEWITHSUNDEEP
c++multithreadingc++11language-lawyerstdthread
Lingxi
Lingxi

Reputation: 14977

Details in the process of constructing a std::thread object

I'm interested in (and confused about) the details of constructing a std::thread object. According to cppreference, both the thread function and all arguments are value-copied to some thread-accessible storage, and then invoke.

1) What exactly is this thread-accessible storage? Is it semantically equivalent to some kind of thread-local storage, and the variables are destructed after the thread function returned?

2) What is the value-category of the arguments when passed to the thread function? The description on cppreference suggests that they are passed as l-values (they are given names anyway). My tests on GCC and clang seem to suggest the opposite, i.e., r-values. Specifically, the following code does not compile:

void f(int& a) {
  std::cout << ++a << '\n';
}

int main() {
    std::thread t(&f, 1);
    t.join();
    return 0;
}

It compiles if we change f to

void f(int&& a) {
  std::cout << ++a << '\n';
}

int main() {
    std::thread t(&f, 1);
    t.join();
    return 0;
}

So, what does the standard say about this?

Upvotes: 6

Views: 1273

Answers (1)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

1) This "thread-accessible storage" bit of text is not represented directly in the standard. The standard simply says that the function is invoked with arguments obtained by decay_copy.

2) If you study decay_copy closely, you will find that it returns by value (because its return type is std::decay of something). So the function f is called with rvalue arguments (prvalue arguments, in fact).

If you want to pass lvalues (references), you can use std::ref and std::cref to wrap them.

The exact quote, C++11 30.3.1.2/4:

Effects: Constructs an object of type thread. The new thread of execution executes INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...) with the calls to DECAY_COPY being evaluated in the constructing thread. Any return value from this invocation is ignored. [ Note: This implies that any exceptions not thrown from the invocation of the copy of f will be thrown in the constructing thread, not the new thread. —end note ] If the invocation of INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...) terminates with an uncaught exception, std::terminate shall be called.

DECAY_COPY is defined in 30.2.6/1:

In several places in this Clause the operation DECAY_COPY(x) is used. All such uses mean call the function decay_copy(x) and use the result, where decay_copy is defined as follows:

template <class T> typename decay<T>::type decay_copy(T&& v)
{ return std::forward<T>(v); }

INVOKE is defined in 20.8.2 pretty much in the same way as cppreference describes the invocation in the link you've provided.

Upvotes: 3

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