Initializing C++11 threads from within an if statement - c++

Question

I am implimenting threads in C++11 and experienceing a compilation issue whenever I initiate a thread from within an if statement.
The error I am receiving is:
file.cpp: In function ‘int main(int, char**)’:
file.cpp:16:2: error: ‘thread1’ was not declared in this scope
thread1.join();
When I move the thread outside of an if statement everything compiles and runs fine.
I am using g++ version 4.8.2 and using the -std=c++11 compiler option.
This code will not compile
#include <unistd.h>
#include <thread>
#include <iostream>
void testthread() {
std::cout << "Thread was run" << std::endl;
}
int main(int argc, char**argv) {
if (true) {
std::thread thread1(testthread);
}
sleep(1);
thread1.join();
return 0;
}
This code compiles and runs as expected
#include <unistd.h>
#include <thread>
#include <iostream>
void testthread() {
std::cout << "Thread was run" << std::endl;
}
int main(int argc, char**argv) {
std::thread thread1(testthread);
sleep(1);
thread1.join();
return 0;
}

Answer 1

The body of an if() statement is a block scope so the lives any variables created within it are bound to its scope. This means that thread1 isn't accessible outside of the if() statement.
Instead you can default construct the thread and then assign it to a new one:
std::thread thread1;
if (true) {
thread1 = std::thread(testthread)
}

Answer 2

You are declaring the thread variable inside the if block. It is only visible there.
If you really need to initialize it inside the if block and use it outside, you can use a pointer and allocate it inside the if block.
std::thread* pThread1 = nullptr;
if (true) {
pThread1 = new std::thread(testthread);
}
sleep(1);
pThread1->join();
delete(pThread1);