使用外部定义并防止重复定义

Use of extern and preventing duplicate definitions

本文关键字：定义 外部      更新时间：2023-10-16

我正在用我正在使用的实际文件重新措辞(但大大剥离了。我需要在半睡半醒时退出发帖。

测试.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"   */

测试.cpp读作：

#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();
}

测试2.cpp读取

/*
* test2.cpp
*
*  Created on: Apr 1, 2019
*      Author: Mike
*/
#include "test.hpp";
void testFunc(){
encoder.getPosition();
//do some other stuff.
}

编译上述内容会给我以下错误：

Linking .pioenvsunofirmware.elf
.pioenvsunosrctest2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `u8x8'
.pioenvsunosrctest.cpp.o (symbol from plugin):(.text+0x0): first defined here
.pioenvsunosrctest2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `button'
.pioenvsunosrctest.cpp.o (symbol from plugin):(.text+0x0): first defined here
.pioenvsunosrctest2.cpp.o (symbol from plugin): In function `u8x8':
(.text+0x0): multiple definition of `encoder'
.pioenvsunosrctest.cpp.o (symbol from plugin):(.text+0x0): first defined here
collect2.exe: error: ld returned 1 exit status
*** [.pioenvsunofirmware.elf] Error 1

如果我尝试使用 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，然后进行测试.cpp没有 extern，我收到此错误：

In file included from srctest.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);
^
srctest.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 srctest.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);
^
srctest.cpp:9:17: error: redefinition of 'OneButton button'
OneButton button(SWITCH, true);
^
In file included from srctest.cpp:2:0:
include/test.hpp:35:18: note: 'OneButton button' previously declared here
extern OneButton button(SWITCH, true);
^
srctest.cpp:10:22: error: redefinition of 'RotaryEncoder encoder'
RotaryEncoder encoder(R1, R2);
^
In file included from srctest.cpp:2:0:
include/test.hpp:36:22: note: 'RotaryEncoder encoder' previously declared here
extern RotaryEncoder encoder(R1, R2);
^
In file included from srctest2.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);
^
srctest2.cpp:10:46: warning: 'u8x8' initialized and declared 'extern'
extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/SCL, /* data=*/SDA, /* reset=*/OLED_RESET);
^
srctest2.cpp:10:46: error: redefinition of 'U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8'
In file included from srctest2.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);
^
srctest2.cpp:11:24: warning: 'button' initialized and declared 'extern'
extern OneButton button(SWITCH, true);
^
srctest2.cpp:11:24: error: redefinition of 'OneButton button'
In file included from srctest2.cpp:8:0:
include/test.hpp:35:18: note: 'OneButton button' previously declared here
extern OneButton button(SWITCH, true);
^
srctest2.cpp:12:29: warning: 'encoder' initialized and declared 'extern'
extern RotaryEncoder encoder(R1, R2);
^
srctest2.cpp:12:29: error: redefinition of 'RotaryEncoder encoder'
In file included from srctest2.cpp:8:0:
include/test.hpp:36:22: note: 'RotaryEncoder encoder' previously declared here
extern RotaryEncoder encoder(R1, R2);
^
*** [.pioenvsunosrctest.cpp.o] Error 1
*** [.pioenvsunosrctest2.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 test.cpp 和 test2.cpp 中使用 extern 会给我几乎相同的错误。

你想要一个在标题中带有extern的声明。只是宣言。所以在测试中

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

成为

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8;

强制性标准报价

当您extern贴在上面并离开初始化时，

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

该extern被有效地忽略，您将获得定义而不是声明。编译器可能会警告您这一点，它确实

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，则该消息将无济于事。无论如何，不要忽视警告。他们是编译器告诉你，虽然编译的东西(语法正确)，但它可能并不意味着你想要什么或做你想做什么(它可能在逻辑上不正确)。您可能会发现警告通常包含比实际错误更有用的信息，因此请找出它们的含义并消除它们。

回到正题，

extern U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8;

在test.hpp中，承诺u8x8已经或将在其他地方定义，并且可以安全使用。下一步信守承诺。在测试中.cpp XOR 测试2.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。

更多阅读

何时在C++中使用外部

和 C，不是C++，但 C 和 C++ 在这里足够接近，因此这个更长的讨论很有用：如何使用 extern 在源文件之间共享变量？

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