全局const初始化以及.h或.cpp文件中的构造函数之间的差异

Question

I was wondering why sometimes my global const defined in seperate .h file isn't properly initialized when I need it. Some tests lead me to situation that I can't understand. I don't know how to explain it, so here's the code:

main.cpp

#include <iostream>
#include "class.h"
using namespace std;

A a;
B b;

int main(int argc, char* argv[]){
A aa;
B bb;
cout<<a.a<<" "<<aa.a<<endl;
cout<<b.b<<" "<<bb.b<<endl;
return 0;
}

class.h

#ifndef CLASS_H
#define CLASS_H
#include "const.h"

class A {
public:
A();
float a;
};

class B {
public:
B():b(CONST){}
float b;
};
#endif

class.cpp

#include "class.h"
A::A()
: a(CONST){}

const.h

#ifndef CONST_H
#define CONST_H
#include <limits>
using namespace std;

const float CONST = numeric_limits<float>::has_infinity ? 
        -numeric_limits<float>::infinity() : 
        -numeric_limits<float>::max();
#endif

After running above code I get:

0 -1.#INF
-1.#INF -1.#INF

when actually I would like to get 4 times '-1.#INF'. Why does it happen this way? If CONST would be '1' instead of above formula, it would work perfectly.

I can "fix" it by making static getConst() method:

static float getConst(){
static const float CONST = numeric_limits<float>::has_infinity ? 
        -numeric_limits<float>::infinity() : 
        -numeric_limits<float>::max();
return CONST;}

but it just doesn't "feel" right. On other hand I just need two of those above... But maybe there's some other way?

And, most importantly, why class B gets "right" CONST and class A don't?

Answer 1

The order of initialization of global object in different translation unit is not guarantee.

Please have a look at this stackoverflow question: Static variables initialisation order

Answer 2

Constant objects have internal linkage, so you get a separate copy of CONST in each compilation unit that includes const.h . In this case, you'll get one in main.cpp , and one in class.cpp . You also have your two global objects in main.cpp .

C++ does not define the order in which global objects in different compilation units are initialised. A s constructor is called from main.cpp , and needs the global constant from class.cpp , which may not yet be initialised. In your particular case, it hasn't been, so you get the incorrect value you saw. B s constructor is inline, so it uses the global constant from main.cpp , which has been initialised since it is defined earlier in the same compilation unit.

By making the constant a function-static object, it is now guaranteed to be initialised when that function is first called.

Answer 3

I don't see the contents of const.h anywhere so I can only guess what CONST is (a macro?).

That aside, in your code example, the member variable A::a is not initialized. Hence, it could have any value - it's uninitialized.

Answer 4

如果我正确理解了您的问题，那么我相信问题是因为未定义任何全局对象的初始化顺序。