c++ - extern的使用和防止重复定义

Question

我正在用我正在使用的实际文件改写这个(但被大大剥离了。而且我需要在我半睡半醒的时候停止发帖。)

Test.hpp 内容如下:

/*
 * test.hpp
 *
 *  Created on: Apr 1, 2019
 *      Author: Mike
 */

#ifndef INCLUDE_TEST_HPP_
#define INCLUDE_TEST_HPP_
#include <U8x8lib.h>



#define R1 13
#define RGROUND 12  //the rotary switch is connected via header pins on the board for development.
#define R2 14
#define SWITCH 27
#define SCL 15
#define SDA 4
#define OLED_RESET 16


#ifdef U8X8_HAVE_HW_SPI
#include <SPI.h>
#endif
#ifdef U8X8_HAVE_HW_I2C
#include <Wire.h>
#endif

#include <RotaryEncoder.h>
#include "OneButton.h"


U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
OneButton button(SWITCH, true);
RotaryEncoder encoder(R1, R2);

void testFunc();

#endif /* INCLUDE_TEST_HPP_ */
/*      end of test.hpp"   */

test.cpp 内容如下:

#include <Arduino.h>
#include "test.hpp"

#include <U8x8lib.h>
#include <RotaryEncoder.h>
#include "OneButton.h"

void setup() {
    u8x8.begin();
    u8x8.clearDisplay();
//  encoder.begin();    //there's no begin functions for either of these.
//  button.begin();
}

void loop() {
    encoder.tick();
    button.tick();
    u8x8.print("starting loop");
    testFunc();
}

test2.cpp 读取

/*
 * test2.cpp
 *
 *  Created on: Apr 1, 2019
 *      Author: Mike
 */

#include "test.hpp";

void testFunc(){
    encoder.getPosition();
    //do some other stuff.
}

编译上面的代码会出现以下错误:

Linking .pioenvs\uno\firmware.elf
.pioenvs\uno\src\test2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `u8x8'
.pioenvs\uno\src\test.cpp.o (symbol from plugin):(.text+0x0): first defined here
.pioenvs\uno\src\test2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `button'
.pioenvs\uno\src\test.cpp.o (symbol from plugin):(.text+0x0): first defined here
.pioenvs\uno\src\test2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `encoder'
.pioenvs\uno\src\test.cpp.o (symbol from plugin):(.text+0x0): first defined here
collect2.exe: error: ld returned 1 exit status
*** [.pioenvs\uno\firmware.elf] Error 1

如果我尝试用 extern 添加这三行

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
extern OneButton button(SWITCH, true);
extern RotaryEncoder encoder(R1, R2);

到 test.hpp 和 test2.cpp 文件，然后到 test.cpp 没有外部，我得到这个错误:

In file included from src\test.cpp:2:0:
include/test.hpp:34:46: warning: 'u8x8' initialized and declared 'extern'
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
include/test.hpp:35:24: warning: 'button' initialized and declared 'extern'
extern OneButton button(SWITCH, true);
^
include/test.hpp:36:29: warning: 'encoder' initialized and declared 'extern'
extern RotaryEncoder encoder(R1, R2);
^
src\test.cpp:8:39: error: redefinition of 'U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8'
U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
In file included from src\test.cpp:2:0:
include/test.hpp:34:42: note: 'U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8' previously declared here
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
src\test.cpp:9:17: error: redefinition of 'OneButton button'
OneButton button(SWITCH, true);
^
In file included from src\test.cpp:2:0:
include/test.hpp:35:18: note: 'OneButton button' previously declared here
extern OneButton button(SWITCH, true);
^
src\test.cpp:10:22: error: redefinition of 'RotaryEncoder encoder'
RotaryEncoder encoder(R1, R2);
^
In file included from src\test.cpp:2:0:
include/test.hpp:36:22: note: 'RotaryEncoder encoder' previously declared here
extern RotaryEncoder encoder(R1, R2);
^
In file included from src\test2.cpp:8:0:
include/test.hpp:34:46: warning: 'u8x8' initialized and declared 'extern'
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
include/test.hpp:35:24: warning: 'button' initialized and declared 'extern'
extern OneButton button(SWITCH, true);
^
include/test.hpp:36:29: warning: 'encoder' initialized and declared 'extern'
extern RotaryEncoder encoder(R1, R2);
^
src\test2.cpp:10:46: warning: 'u8x8' initialized and declared 'extern'
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
src\test2.cpp:10:46: error: redefinition of 'U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8'
In file included from src\test2.cpp:8:0:
include/test.hpp:34:42: note: 'U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8' previously declared here
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
src\test2.cpp:11:24: warning: 'button' initialized and declared 'extern'
extern OneButton button(SWITCH, true);
^
src\test2.cpp:11:24: error: redefinition of 'OneButton button'
In file included from src\test2.cpp:8:0:
include/test.hpp:35:18: note: 'OneButton button' previously declared here
extern OneButton button(SWITCH, true);
^
src\test2.cpp:12:29: warning: 'encoder' initialized and declared 'extern'
extern RotaryEncoder encoder(R1, R2);
^
src\test2.cpp:12:29: error: redefinition of 'RotaryEncoder encoder'
In file included from src\test2.cpp:8:0:
include/test.hpp:36:22: note: 'RotaryEncoder encoder' previously declared here
extern RotaryEncoder encoder(R1, R2);
^
*** [.pioenvs\uno\src\test.cpp.o] Error 1
*** [.pioenvs\uno\src\test2.cpp.o] Error 1
 [ERROR] Took 3.51 seconds 

将这三个放入没有 extern 的 test.hpp 中，然后将它们放入带有 extern 的 test.cpp 和 test2.cpp 中，我得到了几乎相同的错误。

Answer 1

你想要一个在 header 中带有 extern 的声明。只是声明。所以在 Test.hpp 中

U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);

成为

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8;

Obligatory Standard quote

当你在其上粘贴extern并离开初始化时，

// bad code! Do not use!
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8( ... );
                                             ^ initialization   

extern 被有效忽略，你得到一个 definition instead of a declaration .编译器可能会就此向您发出警告，它确实做到了

include/test.hpp:34:46: warning: 'u8x8' initialized and declared 'extern'

但是，如果您不知道自己在寻找什么，那么该消息就无济于事。无论如何，不​​要忽视警告。它们是编译器告诉你，虽然某些东西可以编译(它在语法上是正确的)，但它可能并不意味着你想要什么或做你想做的事(它可能在逻辑上是不正确的)。您可能会发现警告通常包含比导致的实际错误更有用的信息，因此找出它们的含义并消除它们。

回到主题，

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8;

在 test.hpp 中 promise u8x8 已经或将在其他地方定义并且可以安全使用。下一步信守 promise 。在 test.cpp XOR test2.cpp(两者之一，而不是两者)添加

U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);

对 button 和 encoder 做同样的事情。

多读书

When to use extern in C++

和 C，不是 C++，但 C 和 C++ 在这里足够接近，因此这个较长的讨论很有用:How do I use extern to share variables between source files?