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#include <iostream>
#include <fstream>
#include <cmath>
#include <string>
#include <thread>
#include <mutex>
using namespace std;
 
struct Foo {
private:
	int _i = 0;
	std::mutex _mutex;
public:
	int f() const
	{
		std::lock_guard<std::mutex> locker(_mutex);
		return _i;
	}
 
};
 
int main()
{
	Foo foo;
	foo.f();
	cin.get();
	return 0;
}

prog.cpp: In member function 'int Foo::f() const':
prog.cpp:16:44: error: no matching function for call to 'std::lock_guard<std::mutex>::lock_guard(const std::mutex&)'
   std::lock_guard<std::mutex> locker(_mutex);
                                            ^
prog.cpp:16:44: note: candidates are:
In file included from prog.cpp:6:0:
/usr/include/c++/4.9/mutex:379:7: note: std::lock_guard<_Mutex>::lock_guard(std::lock_guard<_Mutex>::mutex_type&, std::adopt_lock_t) [with _Mutex = std::mutex; std::lock_guard<_Mutex>::mutex_type = std::mutex]
       lock_guard(mutex_type& __m, adopt_lock_t) : _M_device(__m)
       ^
/usr/include/c++/4.9/mutex:379:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.9/mutex:376:16: note: std::lock_guard<_Mutex>::lock_guard(std::lock_guard<_Mutex>::mutex_type&) [with _Mutex = std::mutex; std::lock_guard<_Mutex>::mutex_type = std::mutex]
       explicit lock_guard(mutex_type& __m) : _M_device(__m)
                ^
/usr/include/c++/4.9/mutex:376:16: note:   no known conversion for argument 1 from 'const std::mutex' to 'std::lock_guard<std::mutex>::mutex_type& {aka std::mutex&}'

Standard output is empty