如何为Arduino库使用包含保护？

Question

为什么在Example.h中将消息打印两次？ #pragma once是否应该#pragma once阻止它？

Example.h：

#pragma once
#pragma message "Included"

Example.cpp:
#include "Example.h"

Test.ino：

#include "Example.h"

void setup() {}
void loop() {}

输出：

$ ~/Repositories/arduino-1.8.7/arduino --board arduino:avr:mega:cpu=atmega2560 --verify test/Test.ino`
Picked up JAVA_TOOL_OPTIONS: 
Loading configuration...
Initializing packages...
Preparing boards...
Verifying...
In file included from /home/ToBeReplaced/Test/test/Test.ino:1:0:
/home/ToBeReplaced/Arduino/libraries/example/Example.h:2:17: note: #pragma message: Included
 #pragma message "Included"
                 ^
In file included from /home/ToBeReplaced/Arduino/libraries/example/Example.cpp:1:0:
/home/ToBeReplaced/Arduino/libraries/example/Example.h:2:17: note: #pragma message: Included
 #pragma message "Included"
                 ^
Sketch uses 656 bytes (0%) of program storage space. Maximum is 253952 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 8183 bytes for local variables. Maximum is 8192 bytes.

Answer 1

包含防护可防止将同一标头多次包含在同一翻译单元/代码文件中。
两个文件，每个文件都是分开编译的，每个文件都包含带有消息的标头，每个文件在编译时都会显示该消息。

如果您再次将标头和包含保护的标头包含在同一个文件中，那么onyl仍然会对该文件显示一次消息。