knownConditionTrueFalse false positive

[Nikita Leontiev]

cppcheck 2.17.1 generates knownConditionTrueFalse false positive for double-checked locking singleton realization:

#include <mutex>

class ScopedLock
{
private:
    std::mutex *m_lock;
public:
    explicit ScopedLock(std::mutex *lock) : m_lock(lock)
    {
        m_lock -> lock();
    }
    ~ScopedLock()
    {
        m_lock -> unlock();
    }
};

class Singleton
{
private:
    static Singleton *m_instance;
    static std::mutex m_lock;
public:
    static Singleton *get_instance()
    {
        if (!m_instance)
        {
            ScopedLock lock(&m_lock);
            if (!m_instance)
                m_instance = new Singleton();
        }
        return m_instance;
    }
    static void free_instance()
    {
        if (m_instance)
        {
            delete m_instance;
            m_instance = 0;
        }
    }
};

Singleton *Singleton::m_instance = 0;
std::mutex Singleton::m_lock;

test\main.cpp:29:8: style: Condition '!m_instance' is always true [knownConditionTrueFalse]
   if (!m_instance)
       ^
test\main.cpp:26:7: note: Assuming that condition '!m_instance' is not redundant
  if (!m_instance)
      ^
test\main.cpp:29:8: note: Condition '!m_instance' is always true
   if (!m_instance)
       ^

[CHR]

cppcheck does not consider multi-threaded execution.

[Nikita Leontiev]

cppcheck behavior is inconsistent, because it doesn't generate same warning for the code below:

#include <mutex>

class Singleton
{
private:
    static Singleton *m_instance;
    static std::mutex m_lock;
public:
    static Singleton *get_instance()
    {
        if (!m_instance)
        {
            m_lock.lock();
            if (!m_instance)
                m_instance = new Singleton();
            m_lock.unlock();
        }
        return m_instance;
    }
    static void free_instance()
    {
        if (m_instance)
        {
            delete m_instance;
            m_instance = 0;
        }
    }
};

Singleton *Singleton::m_instance = 0;
std::mutex Singleton::m_lock;

[CHR]

Thanks for reporting, ticket is here: https://trac.cppcheck.net/ticket/13675