Visual Studio 2013 std::thread - c++

Question

the following program give some strange compile/run-time behavior when compiled with Visual Studio 2013:
#include "stdafx.h"
#include <thread>
#include <chrono>
#include <iostream>
int main()
{
{//works
std::thread([](){
std::cout << " thread running1\n";
});
}
{//compile but synstax error exist
auto func = [](){
std::cout << " thread running2\n";
};
std::thread(fun); //fun is not defined
}
{//compile, see next block for mystery compile error
auto func = [](){
std::cout << " thread running2\n";
};
std::thread tmp(func);
}
{//does not compile and don't know why
auto func = [](){
std::cout << " thread running2\n";
};
std::thread(func); //error C2371: 'func' : redefinition; different basic types
}
return 0;
}
When this program work, there may crash as there is race condition between the thread. The main thread may end before the other threads.
Does anybody know why the second block and last block does not work?

Answer 1

{//compile but synstax error exist
auto func = [](){
std::cout << " thread running2\n";
};
std::thread(fun); //fun is not defined
}
There's no syntax error here, std::thread(fun) default constructs an std::thread object named fun.
The error in the last block is because of the same reason
std::thread(func); //error C2371: 'func' : redefinition; different basic types
You're trying to default construct an std::thread object named func above and that's an error because a lambda of the same name already exists in the same scope. To pass the lambda to the thread constructor use braces instead
std::thread{func};
Now, after you've made these changes your code will compile but will fail at runtime because the thread objects in blocks 1, 3 & 4 are all going to call std::terminate (of course, your program terminates when the first thread object calls std::terminate so the other two doing that is a moot point).
The reason that happens is that you have joinable threads in all 3 blocks and if the destructor of such a thread object runs, std::terminate will be called. To avoid that you must call thread::join (you could also call thread::detach but don't do that).
For instance
{//works
std::thread([](){
std::cout << " thread running1\n";
}).join();
}
Live demo