PTHREAD_MUTEX_INITIALIZER inside C++ member function cannot compile?

Question

class A {
    public:
        A();
    private:
        pthread_mutex_t mu;
};

A::A()
{
    mu = PTHREAD_MUTEX_INITIALIZER;  //cannot compile
}

Ca

Answer 1

Instead of this:

A::A()
{
    mu = PTHREAD_MUTEX_INITIALIZER;  //cannot compile
}

Try this:

A::A()
{
pthread_mutex_init( &(mu), NULL);
}

The PTHREAD_MUTEX_INITIALIZER is a macro,a C struct initializer for something like {0,0,0,0,0{0}} and can only be used at the point of definition.

Answer 2

Even if we change this to use an initializer list in the constructor it still fails:

#include <pthread.h>

struct foo {
  pthread_mutex_t test;
  foo() : test(PTHREAD_MUTEX_INITIALIZER) {}
};

int main() {
  foo f;
}

We can see why it fails and an only be used for initialisation in a few contexts by looking at the output from the pre-processsor:

struct foo {
  pthread_mutex_t test;
  foo() : test({ { 0, 0, 0, 0, 0, { 0 } } }) {}
};

It's not legal to use nested braces for initialisation like that in C++03, but what's more interesting perhaps is that C++11 makes this syntax and usage perfectly legal.

In your original code we can see a few more things:

A::A()
{
    const pthread_mutex_t test = PTHREAD_MUTEX_INITIALIZER; //  initialization - fine
    mu = test; // assignment - fine
    mu = PTHREAD_MUTEX_INITIALIZER;  // assignment - C++11 only
}

Answer 3

I like the answers from @askmish & @Diego. I like also what @Flexo explained.

But just as an option, if you are intent on initializing with the PTHREAD_MUTEX_INITIALIZER macro, what you can do is, make the mutex declaration inside the class definition static like this:

class A {
    public:
        A();
    private:
        static pthread_mutex_t mu;
};

And then you can initialize this static mutex in your source file BUT outside any member function, like this:

class A {
    public:
        A();
    private:
        static pthread_mutex_t mu;
};

pthread_mutex_t A::mu = PTHREAD_MUTEX_INITIALIZER;

A::A()
{
}

Your options:

• So either you keep the macro and go static (as shown here). OR,
• you keep the declaration of the mutex the same (non-static) but use the pthread_mutex_init() function (as others have indicated) inside your member-function.

Answer 4

Use pthread_mutex_init in this case, as the constant is for compile-time initialization.