C - C++ Threads - Important Considerations - Thread function arguments are always passed by value

NOTE: Arguments are always copies into the internal storage for threads.

This means that any changes made by the thread to the arguments passed does not affect the original arguments.

The compiler may not even allow the compilation of the program.
To fix, ensure that arguments are passed by reference.

Example

#include <iostream>
#include <string>
#include <thread>
#include <functional>
 
 
void ChangeTotal(std::int& targetTotal)
{
  targetTotal = 20;
  std::cout << "Changing target_total to " << targetTotal << std::endl;
}
 
int main()
{
  int total = 10;
  std::thread t1(ChangeTotal, total);
  t1.join();
  std::cout << "Current Total is " << total << std::endl;
  return 0;
}

NOTE: Trying to compile this will fail with a message such as the following.

The reason for this is that the arguments being passed to ChangeTotal() are not being passed by reference.

/usr/include/c++/9/thread: In instantiation of ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args = {int&}; <template-parameter-1-3> = void]’:
test6.cpp:17:36:   required from here
/usr/include/c++/9/thread:120:44: error: static assertion failed: std::thread arguments must be invocable after conversion to rvalues
  120 |           typename decay<_Args>::type...>::value,
      |                                            ^~~~~
/usr/include/c++/9/thread: In instantiation of ‘struct std::thread::_Invoker<std::tuple<void (*)(int&), int> >’:
/usr/include/c++/9/thread:131:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args = {int&}; <template-parameter-1-3> = void]’
test6.cpp:17:36:   required from here
/usr/include/c++/9/thread:243:4: error: no type named ‘type’ in ‘struct std::thread::_Invoker<std::tuple<void (*)(int&), int> >::__result<std::tuple<void (*)(int&), int> >’
  243 |    _M_invoke(_Index_tuple<_Ind...>)
      |    ^~~~~~~~~
/usr/include/c++/9/thread:247:2: error: no type named ‘type’ in ‘struct std::thread::_Invoker<std::tuple<void (*)(int&), int> >::__result<std::tuple<void (*)(int&), int> >’
  247 |  operator()()
      |  ^~~~~~~~

Solution

Ensure that arguments are passed by reference:

#include <iostream>
#include <string>
#include <thread>
#include <functional>
 
 
void ChangeTotal(int& targetTotal)
{
  targetTotal = 20;
  std::cout << " Changing targetTotal to " << targetTotal << std::endl;
}
 
int main()
{
  int total = 10;
  std::thread t1(ChangeTotal, std::ref(total));
  t1.join();
  std::cout << "Total is " << total << std::endl;
  return 0;
}

NOTE: The std::ref changes the parameter to be passed by reference.

Wiki

Table of Contents

C - C++ Threads - Important Considerations - Thread function arguments are always passed by value

Example

Solution