如何為Arduino庫使用包含保護？

Question

為什么在Example.h中將消息打印兩次？ #pragma once是否應該#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() {}

輸出：

$ ~/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

包含防護可防止將同一標頭多次包含在同一翻譯單元/代碼文件中。
兩個文件，每個文件都是分開編譯的，每個文件都包含帶有消息的標頭，每個文件在編譯時都會顯示該消息。

如果您再次將標頭和包含保護的標頭包含在同一個文件中，那么onyl仍然會對該文件顯示一次消息。