How to pass struct elements to asynchronous threads? - c++

Question

I am trying to launch a function threaded using launch::async. However, I noticed that this doesn't work when passing struct elements as parameter:
The code
#include <future>
#include <vector>
#include <thread>
struct example { int ten; };
void threadFunction(int number, std::string hi) {
printf("%s Your number is %d\n", hi.c_str(), number + 1);
}
int main() {
example ex;
ex.ten = 9;
std::string someString = "Hi there!";
std::vector< std::future< void > > threads(5);
for (uint16_t s = 0; s < 5; s += 1) {
threads[s] = async(std::launch::async,
[ex.ten,
someString] {
threadFunction(ex.ten, someString);
});
}
}
gives the following errors:
file.cpp: In function ‘int main()’:
file.cpp:25:39: error: expected ‘,’ before ‘.’ token
[ex.ten,
^
file.cpp:25:39: error: expected identifier before ‘.’ token
file.cpp:25:43: error: expected ‘]’ before ‘,’ token
[ex.ten,
^
file.cpp: In lambda function:
file.cpp:25:43: error: expected ‘{’ before ‘,’ token
file.cpp: In function ‘int main()’:
file.cpp:26:46: error: expected ‘)’ before ‘]’ token
someString] {
^
file.cpp:28:8: error: expected primary-expression before ‘)’ token
});
When replacing ex.ten with some other variable ten it does work.
So my questions are:
1. Why does launch::async not work with struct elements?
2. Is there a way to do it in a more elegant way than to make a variable for each element in the struct and pass those variables instead? (such as int ten = ex.ten; etc)

Answer 1

You can't pass single struct field that way into lambda in C++. The error you get is not connected to std::launch or communicating between threads. What you can do instead is:
C++11 - copy field into local variable and capture that variable:
auto ten = ex.ten;
threads[s] = async(std::launch::async,
[ten,
someString] {
threadFunction(ten, someString);})
C++14 and later - initialize variable in capture list:
threads[s] = async(std::launch::async,
[ten = ex.ten,
someString] {threadFunction(ten, someString);})

Answer 2

What you are trying to do is to capture a field in the struct in lambda. This is not possible, you can only capture the whole struct.
It has nothing to do with std::launch, it's a basic property of lambda closure.
In C++14, you can have captures with initializers, which might be what you want:
auto lam = [ten = ex.ten]...