Why can\'t I pass a non-const reference to `std::async` and how do I work around it?

Incompatibilities in Argument Passing to std::async by Reference

The inability to pass a non-const reference as an argument to std::async has perplexed developers. Consider the following example:

<code class="cpp">#include <functional>
#include <future>

void foo(int& value) {}

int main() {
    int value = 23;
    std::async(foo, value);
}</future></functional></code>

Compilers report an error with this code:

error: no type named ‘type’ in ‘class std::result_of<void>()’</void>

Encasing the argument in std::reference_wrapper resolves the issue, but it raises questions about the underlying mechanism.

Reasoning Behind Value-Based Argument Passing

std::async's design intentionally defaults to passing all arguments by value. This choice ensures safety, as copies of arguments cannot become dangling or exhibit race conditions.

Addressing Non-Const Reference Parameters

However, there are scenarios where passing an argument by non-const reference is necessary. This is where std::ref comes into play. std::ref allows developers to explicitly opt into the risky semantics of passing non-const references, acknowledging their understanding of the potential consequences.

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