qt中undefined reference to CON(CON是定义的结构体)

报错：error: undefined reference to `CON'
头文件caltulate.h

#ifndef CALTULATE_H
#define CALTULATE_H
# include "stdio.h"
# include "math.h"
# include "stdlib.h"
# include "string.h"
# define pai 3.1415926
struct condenser2 // 冷凝器
{
    double in_diam_i;
    double in_diam_o;
    double in_wall_width;
    double out_diam_i;
    double out_diam_o;
    double out_wall_width;
    double length;
};
extern struct condenser2 CON;
#endif // CALTULATE_H

定义结构体的a.cpp

extern "C"{
#include "caltulate.h"
}
CON.in_diam_i = cond_in_d_i;
    CON.in_diam_o = cond_in_d_o;
    CON.in_wall_width = cond_in_wall_w;
    CON.out_diam_i = cond_out_d_i;
    CON.out_diam_o = cond_out_d_o;
    CON.out_wall_width = cond_out_wall_w;
    CON.length = cond_length;

调用结构体的caltulate.cpp

#include<iostream>
#include<QFile>
#include<QDebug>
#include<QTextStream>

#include<typeinfo>
extern "C"{
#include "caltulate.h"
}

double x = CON.in_diam_i;