How to define a "global" struct? - c++

Question

Let say I've decleared this within MyTools.h
#ifndef _MYTOOLS_
#define _MYTOOLS_
typedef struct {
// const
double LN20;
double LN40;
// methods
double NoteToFrequency(int noteNumber);
} Tool;
extern const Tool tool;
#endif // !_MYTOOLS_
For every compilation unit, there is only a global/const/unique instance of Tool. Exactly what I want.
But now: how can I define it? In the .h i've only declared it. How can I define it in .cpp? Tried somethings like:
tool.LN20 = 1.34;
But of course it doesn't works. And the method's definition?

Answer 1

extern doesn't define any variable it just declares it. What you wan't to achieve can be done as below:
The link Global const object shared between compilation units explains how to do it with extern const
t.h file
#ifndef _MYTOOLS_
#define _MYTOOLS_
struct Tool {
// const
double LN20;
double LN40;
double NoteToFrequency(int noteNumber);
} ;
extern const Tool tool ;
#endif // !_MYTOOLS_
t1.cpp
#include "t.h"
#include <stdio.h>
void use_tool()
{
printf("%f\n",tool.LN20);
printf("%f\n",tool.LN40);
return;
}
t2.cpp
#include "t.h"
#include <stdio.h>
const Tool tool = {.LN20 = 20.0, .LN40 = 30.2};
double Tool::NoteToFrequency(int noteNumber)
{
return 12.0;
}
void use1_tool()
{
printf("%f\n",tool.LN20);
printf("%f\n",tool.LN40);
return;
}
int main()
{
void use_tool();
use_tool();
use1_tool();
return 0;
}
Hope this helps.