使用extern并防止重复定义

时间：2019-04-01 23:29:00

标签： c++ header extern

我要用我正在使用的实际文件来重新措辞（但已被剥离掉了。半睡着时我需要退出发布）。

Test.hpp读取：

/*
 * 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读为：

#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.
}

编译上面的代码会给我以下错误：

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，然后到没有外部代码的test.cpp，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

在没有extern的情况下将这三个变量放入test.hpp中，然后在使用extern的情况下将其放入test.cpp和test2.cpp中，我得到的错误几乎相同。

1 个答案:

答案 0 :(得分：2)

您想要一个在标题中带有extern的声明。只是宣言。所以在Test.hpp 

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

将extern粘贴在其上并保留初始化时，

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

extern被有效地忽略，您得到一个definition instead of a declaration。编译器可能会对此发出警告，并且确实存在

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

但是，如果您不知道要查找的内容，则该消息没有太大帮助。无论如何，请不要忽略警告。它们是编译器告诉您的，虽然某些东西在编译（在语法上是正确的），但这可能并不意味着您想要什么或做您想做的事情（在逻辑上可能不正确）。您可能会发现警告通常包含比实际错误更多的有用信息，因此请找出警告的含义并消除它们。

回到主题，

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8;
test.hpp中的

承诺u8x8已经或将在其他地方定义并且可以安全使用。下一步将兑现承诺。在test.cpp XOR test2.cpp（两个，不是两个）之一中添加

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

对button和encoder做同样的事情。

更多阅读

When to use extern in C++

和C（不是C ++），但是C和C ++在此处足够接近，因此可以进行较长的讨论：How do I use extern to share variables between source files?

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