Why create an include/ directory in C and C++ projects?

Question

When I work on my personal C and C++ projects I usually put file.h and file.cpp in the same directory and then file.cpp can reference

Answer 1

1 . .hpp and .cpp doesn't necessary have 1 to 1 relationship, there may have multiple .cpp using same .hpp according to different conditions (eg:different environments), for example: a multi-platform library, imagine there is a class to get the version of the app, and the header is like that:

Utilities.h

#include 
class Utilities{
    static std::string getAppVersion();
}

main.cpp

#include Utilities.h
int main(){
    std::cout << Utilities::getAppVersion() << std::ends;
    return 0;
}

there may have one .cpp for each platform, and the .cpp may be placed at different locations so that they are easily be selected by the corresponding platform, eg:

.cpp for iOS (path:DemoProject/ios/Utilities.cpp):

#include "Utilities.h"
std::string Utilities::getAppVersion(){
    //some objective C code
}

.cpp for Android (path:DemoProject/android/Utilities.cpp):

#include "Utilities.h"
std::string Utilities::getAppVersion(){
    //some jni code
}

and of course 2 .cpp would not be used at the same time normally.

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2.

#include "file.h" 

instead of

#include "include/file.h" 

allows you to keep the source code unchanged when your headers are not placed in the "include" folder anymore.