Thread safe initialization of const variable in header file

[kangaba]

Hi,
I'm using c++11 and I wonder if the initialization of the constant variable "Andrew" is thread safe in this code:

//some_file.h
namespace ns {
 
struct Person {
    const int id;
    const QString name;
};
 
const Person Andrew { 2, QStringLiteral("Some string") };
}

//some_file.h
namespace ns {

struct Person {
    const int id;
    const QString name;
};

const Person Andrew { 2, QStringLiteral("Some string") };
}

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[stampede]

You probably want to declare Andrew as "extern" variable and define it in some .cpp file - now you will have one Andrew per every file in which you include your header:

namespace ns {
 
struct Person {
    const int id;
    const QString name;
};
 
extern const Person Andrew;
}
 
// some.cpp
#include "some_file.h"
const ns::Person ns::Andrew = { 2, QStringLiteral("Some string") };

namespace ns {
 
struct Person {
    const int id;
    const QString name;
};
 
extern const Person Andrew;
}

// some.cpp
#include "some_file.h"
const ns::Person ns::Andrew = { 2, QStringLiteral("Some string") };

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Such global data will be initialized before entering main(), so there will be no multiple threads trying to access Andrew at this point.
As a side note, if you want to be sure that something gets called exactly once, you can use std::call_once.